Disclosed is a compound which has an intestinal phosphate transporter (NPT-IIb) inhibition activity and is useful as an active ingredient for a therapeutic agent and/or a prophylactic agent for hyperphosphatemia. A tetrahydrobenzothiophene compound represented by formula (I) has an NPT-IIb inhibition activity and can be used as a prophylactic agent and/or a therapeutic agent for hyperphosphatemia. (In the formula, R1 represents -O-(lower alkyl), -(lower alkylene)-phenyl, or the like; R2 and R3 are same as or different from each other and independently represent H, a lower alkyl group, a cycloalkyl group, an aryl group, a heteroaryl group, or the like, or R2, R3 and a nitrogen atom to which R2 and R3 are bound together may form a 5- to 7-membered saturated cyclic amino group; R4's are same as or different from each other and independently represent a halogen atom, a lower alkyl group, or the like; and n represents 0 to 2.)