An object of the present invention is to provide a drug with a high therapeutic or prophylactic effect on dyskinesia, without accompanying aggravation of symptoms of the primary disease, and with fewer side effects. Accordingly, the present invention provides a therapeutic or prophylactic agent for dyskinesia, comprising as an effective component a compound having a 4,5-epoxy morphinan skeleton, which compound is represented by the Formula (I) below, or a pharmaceutically acceptable acid addition salt thereof: [wherein the double line composed of a dashed line and a solid line represents a double bond or single bond, R 1 is C 4 -C 7 cycloalkylalkyl, R 2 is C 1 -C 5 straight or branched alkyl, and B is -CH=CH-.]
