A method for producing a patch comprising a support layer and an adhesive agent layer comprises:a mixture preparation step of mixing asenapine or a pharmaceutically acceptable salt thereof with sodium acetate whose particle diameter D50 at a cumulative volume of 50% in a particle diameter distribution is 40 to 1000 µm, in such a manner that the sodium acetate and sodium diacetate generated from the sodium acetate have a particle diameter D50 of 10 µm or smaller, thereby obtaining a mixture containing the sodium diacetate and the asenapine or pharmaceutically acceptable salt； andan adhesive-agent-layer formation step of forming the adhesive agent layer comprising the sodium diacetate, the asenapine or pharmaceutically acceptable salt, and a pressure-sensitive adhesive base agent, by using an adhesive agent layer composition obtained by mixing the mixture with the pressure-sensitive adhesive base agent.
