The purpose is to provide a compound which has an intestinal phosphatetransporter (NPT-IIb) inhibitory action and is useful as an active ingredientof an agent fortreating and/or preventing hyperphosphatemia.A tetrahydrobenzothiophene compound of the following formula (I) has NPT-IIBinhibitory action and can be used as an agent for treating and/or preventinghyperphosphatemia:(see formula I)wherein, R1 represents -O-lower alkyl, -lower alkylene-phenyl, or the like； R2andR3 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 nitrogenatom to whichthey bind to form 5- to 7-membered saturated cyclic amino； R4's are the sameas ordifferent from each other and represent halogen, lower alkyl； and n represents0 to 2.
