The present invention provides compounds represented by the formula (Ia): ##STR00001## the formula (Ib): ##STR00002## the formula (Ic): ##STR00003## and the formula (Id): ##STR00004## wherein each symbol is as defined in the specification. According to the present invention, these compounds have a DGAT inhibitory activity and are useful for the prophylaxis, treatment or improvement of diseases or pathologies caused by high expression or high activation of DGAT.