The present invention relates to a phenacylamine derivative of the formula (I) or a salt thereof: wherein A is alkyl, cycloalkyl, phenyl which may be substituted by Y, pyridyl which may be substituted by Y, or pyrazolyl which may be substituted by Y, R1 and R2 are each alkyl, or R1 and R2 may together form a 3- to 6-membered saturated carbocycle, R3 is hydrogen, alkyl, alkoxyalkyl, alkylthioalkyl or COR4, X is halogen, alkyl, haloalkyl, alkenyl, haloalkenyl, alkynyl, haloalkynyl, alkoxy, haloalkoxy, alkenyloxy, haloalkenyloxy, alkynyloxy, haloalkynyloxy, alkylthio, haloalkylthio, alkenylthio, haloalkenylthio, alkynylthio, haloalkynylthio, alkylsulfinyl, haloalkylsulfinyl, alkylsulfonyl, haloalkylsulfonyl, dialkylaminosulfonyl, nitro, cyano, phenyl which may be substituted by Y, phenoxy which may be substituted by Y, benzyloxy which may be substituted by Y, or pyridyloxy which may be substituted by Y, and n is an integer of from 0 to 5.