The present invention provides : a compound (In the formula : R1 and R2 represent a hydrogen atom, a C1-4 alkyl, or the like; n represents 1 or 2; ring A represents a C6-10 aromatic carbon ring or the like; R3, R4 and R5 represent a hydrogen atom, a C1-6 alkyl, a C1-4 alkoxy, or the like; ring B represents a 5- to 10-membered heteroaromatic ring or the like; R6, R7 and R8 represent a hydrogen atom, a C1-6 alkyl, -NRaRb or the like; and Ra and Rb represent a hydrogen atom, a C1-4 alkyl or the like) represented by formula (1' ) which is useful as a therapeutic agent against diseases associated with group II mGlu receptors, or a pharmaceutically acceptable salt thereof; and a pharmaceutical composition containing said compound.