The purpose is to provide a compound which has an intestinal phosphate transporter (NPT-IIb) inhibitory action and is useful as an active ingredient of an agent for treating and/or preventing hyperphosphatemia.A tetrahydrobenzothiophene compound of the following formula (I) has NPT-IIb inhibitory action and can be used as an agent for treating and/or preventing hyperphosphatemia:wherein, R1 represents —O-lower alkyl, -lower alkylene-phenyl, or the like； R2 and R3 are the same as or different from each other and represent H, lower alkyl, cycloalkyl, aryl, heteroaryl or the like, or, R2 and R3 may be combined with a nitrogen atom to which they bind to form 5- to 7-membered saturated cyclic amino； R4's are the same as or different from each other and represent halogen, lower alkyl； and n represents 0 to 2.
