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.