A brassinosteroid metabolism inhibitor, which comprises as an active ingredient a compound represented by the following formula (I) or (II): wherein R1 and R2 independently represent hydrogen atom or a lower alkyl group, R3 represents a hydrogen atom, a lower alkyl group, or a lower alkoxyalkyl group, R4 represents a phenyl group which may be substituted, X represents a single bond or —CH2—, R11 represents a lower alkyl group, a lower alkenyl group or a phenyl group which may be substituted, R12 represents a lower alkyl group or a phenyl group which may be substituted, and R13 represents a phenyl group which may be substituted, or a salt thereof.