Disclosed is a method for treating osteoarthritis comprising a step of administering a patch to a patient. The patch comprises a backing layer and an adhesive layer laminated on the backing layer, the adhesive layer comprises an adhesive base, diclofenac sodium, dimethyl sulfoxide, and an organic acid, and a content of the organic acid is 6 mass % to 8 mass % with respect to a total mass of the adhesive layer. The patch is applied to an affected part once a day, and a dose of diclofenac sodium per administration is 70 mg to 150 mg.