The present invention provides a novel P2X 3 and/or P2X 2 / 3 receptor antago nist. A compound represented by the formula (I): wherein R a , R b and R c are, (a) R a and R b are taken together =Z； and R c is a group represented by R 1c ； or (b) R b and R c are taken together to form a bond； and R a is a group represented by -Y-R 1a ； R 1a and R 1c are each independently hydrogen, substituted or unsubstituted alkyl, etc.； R 2 and R 3 are each independently substituted or unsubstituted aryl, etc.； R 4a and R 4b are each independently hydrogen, substituted or unsubstituted alkyl, etc.； X is -N(R 5 )-, etc.； R 5 is hydrogen, substituted or unsubstituted lower alkyl, etc.； -Y- is -O-, etc.； =Z is =O, etc.； and n is an integer of 0 to 4.
