A compound represented by the formula (I): wherein R1 is lower alkyl, cycloalkyl or aromatic hydrocarbon ring, each of which is optionally substituted with one or more substituents； R2 is hydrogen atom, halogen atom, lower alkyl, halo(lower)alkyl or lower alkoxy； and R3 is (1) a group represented by the formula: wherein R4 is lower alkyl, etc.； (2) a group represented by the formula: wherein R5 is lower alkyl, etc.； (3) a group represented by the formula: wherein R6 is lower alkyl, etc.； or (4) a group selected from halogen atom, carboxy, hydroxy and lower alkoxy, or a salt thereof.
