An object of the present invention is to provide a drug with a hightherapeuticor prophylactic effect on dyskinesia, without accompanying aggravation ofsymptomsof the primary disease, and with fewer side effects.Accordingly, the present invention provides a therapeutic or prophylacticagent for dyskinesia, comprising as an effective component a compound having a4,5-epoxy morphinan skeleton, which compound is represented by the Formula (I)below, or a pharmaceutically acceptable acid addition salt thereof:(see formula I)[wherein the double line composed of a dashed line and a solid line representsadouble bond or single bond, R1 is C4-C7 cycloalkylalkyl, R2 is C1-C5 straightorbranched alkyl, and B is -CH=CH-.]
