The Korean Supreme Court reiterated that, where a patented invention solves a technical problem that had not been solved in the prior art, and a product accused of patent infringement solves the same technical problem by employing essentially the same technical idea, any incidental advantages of the accused product arising from the adoption of widely-used technical means do not, in themselves, amount to a difference sufficient to defeat the applicability of the doctrine of equivalents (Supreme Court Case No. 2022 Hu 10722 rendered on January 29, 2026).

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▶Background of the Case

The patent at issue (“patented invention”) relates to a face mask intended to overcome two problems of conventional masks: the structural complexity arising from separate strap-coupling and strap-length-adjustment mechanisms, and the inability to maintain a stable initial wear state due to the length-adjustment members contacting the back of the wearer’s ears.

To solve these problems, claim 1 recites, inter alia, four close-contact hole portions (“CHPs”) integrally extended from the upper and lower ends of each of the left and right sides of the central cover portion of the mask body, with a strap-pulling separation space (“SPSS”) formed between the upper CHP and the lower CHP on each side. The hanger straps pass through the resulting close-contact holes in a close-contact state and are exposed at the SPSS, enabling a user to adjust the strap-length simply by pulling the exposed portion.

In an affirmative scope confirmation trial filed by the patentee, the accused product was found to share substantially the same overall structure as the patented invention, but with two differences: (i) the accused product’s CHPs are bonded to the front face of the mask body (i.e., the face directed away from the wearer’s skin), whereas the patented invention’s CHPs are bonded to the rear face; and (ii) the accused product’s upper and lower CHPs are connected via a connecting portion in which a partial cut-opening forms the SPSS, whereas the patented invention’s upper and lower CHPs are completely separated from each other.

▶The Doctrine of Equivalents and the Issue

Under Korean patent law, an accused product that differs from a patented claim in some element(s) still falls within the patent’s scope of equivalents where: (i) it employs the same principle for solving the problem as the patented invention; (ii) it achieves substantially the same effects; (iii) the modification(s) would have been easily conceivable by a person of ordinary skill in the art; and (iv) no special circumstances apply.

The principal issue here was whether the accused product achieves “substantially the same effects” as the patented invention. In particular, the question was whether the additional advantages of the accused product (improved wearing comfort, simplified manufacturing, and the formation of a compression portion) amounted to a substantial difference sufficient to preclude a finding of equivalence.

▶IP High Court Decision

The IP High Court held that the accused product fell outside the scope of equivalents. 

The court first found that the core technical idea of the patented invention, i.e., the feature of integrating the strap-coupling and strap-length-adjustment structures by forming CHPs and the SPSS on the mask body, was already disclosed in Korean Patent No. 510164 (“prior art reference”), which also shows a disposable dust mask. Because the core technical idea itself was not novel, the court was of the view that the effects must be compared not by reference to that technical idea as a whole, but by reference to the functions and roles of the individual differing structural elements.

Applying that approach, the IP High Court found that the differing structural elements of the accused product produced three additional effects compared with the patented invention: (i) improved wearing comfort, as the CHPs do not contact the wearer’s skin; (ii) a simplified manufacturing process, as the upper and lower CHPs can be formed at once due to their connected configuration; and (iii) a compression portion, formed by the cut-opening configuration, that presses the strap without touching the skin. Because the accused product achieved these additional effects beyond those of the patented invention, the court found that the two could not be regarded as producing substantially identical effects, and concluded that these differences defeated equivalence.

▶Supreme Court Decision

The Supreme Court reversed the IP High Court decision and remanded the case for further review, for the reason that the lower court had erred in its analysis of the substantial identity of effects. The Court reiterated its established framework that, in assessing the substantial identity of effects, the focus should be placed on whether the accused product also solves the technical problem that the patented invention solved over the prior art. Where the patented invention solves a problem that remained unsolved in the prior art, and the accused product likewise solves that problem, the effects are, in principle, to be regarded as substantially identical. The Court added that the alternative approach of comparing the functions and roles of the individual differing structural elements is reserved for the exceptional circumstance where the core technical idea of the patented invention is found to have been already disclosed in the prior art.

Applying this framework, the Court found that the technical problem solved by the patented invention, i.e., simplifying the mask structure and enabling a stable initial wear state, had not been solved by any prior art of record. Although the prior art reference disclosed forming a groove in a closing portion through which a hanger band could be drawn out to adjust its length, none of the prior art references taught integrally extending the CHPs from the mask body itself to simplify the structure. Accordingly, the Court found the patented invention’s unique problem-solving means as “forming CHPs that are integrally extended from the upper and lower ends of the left and right sides of the central cover portion, and coupling the hanger straps through the resulting close-contact holes in a close-contact state.”

This same problem-solving idea was found to be implemented in the accused product, which likewise integrates the strap-coupling and strap-length-adjustment structures, thereby simplifying the mask structure and enabling a stable initial wear state. As for the three additional effects referred to by the IP High Court, the Supreme Court characterized them as “merely incidental effects arising from the adoption of widely-used technical means,” holding that the mere existence of such incidental effects, in addition to the same effects of the patented invention, does not amount to a substantial difference in effects capable of denying the application of the doctrine of equivalents.

▶Implications

This decision provides important clarification on the “substantial identity of effects” prong of the doctrine of equivalents in Korea. The decision confirms that, where the accused product solves the same technical problem that the patented invention solved over the prior art, the effects of the two inventions are, in principle, to be regarded as substantially identical, even where the accused product exhibits additional effects not achieved by the patented invention. Such additional effects will not preclude a finding of equivalence where they are properly characterized as incidental effects arising from the adoption of widely-used technical means.