A compound represented by the following formula (1): (wherein the carbon atom denoted by * is in the R-configuration; R1 and R2 are each independently a hydrogen atom, a halogen atom, an amino group, a hydroxyl group, a hydroxyamino group, a nitro group, a cyano group, a sulfamoyl group, a C1-C6 alkyl group, a C1-C6 alkoxy group, a C1-C6 alkylsulfanyl group, a C1-C6 alkylsulfinyl group or a C1-C6 alkylsulfonyl group; and A is a substituted or unsubstituted heteroaryl group), or a pharmaceutically acceptable salt thereof.