The present invention provides compounds represented by general formula (I): or pharmaceutical acceptable salts thereof, wherein R1 and R2 are each hydrogen or lower alkyl; R3, R4, R5 and R6 are each hydrogen, halogen, lower alkyl or lower alkoxy; R7 and R8 are each hydrogen, halogen, lower alkyl, halo-lower alkyl, lower alkoxy, cycloalkyl, aryl, heteroaryl, cyano, a hydroxyl group, lower acyl, carboxy or the like; R9 is —C(O)—R10, -A1-C(O)—R10, —O-A2-C(O)—R10 or a tetrazol-5-yl group, which exhibit potent and selective β3-adrenoceptor stimulating activities. The present invention also provides pharmaceutical compositions containing said compound, and uses thereof.