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.).El propósito es proporcionar un compuesto que tiene una acción inhibitoria del transportador (TNP-llb) fosfato intestinal y es útil como ingrediente activo de un agente para tratar y/o prevenir la hiperfosfatemia. Un compuesto de tetrahidrobenzotiofer io de la siguiente fórmula (I) que tiene una acción inhibitoria del TNP-llb y puede utilizarse como un agente para tratar y/o prevenir la hiperfosfatemia: (Ver Formula). donde, R1 representa -O-alquilo inferior, -alquileno-fenilo inferior, o similares; R2 y R3 son iguales o diferentes entre sí y representan H, alquilo inferior, cicloalquilo, arilo, heteroarilo o similares; o R2 y R3 que pueden combinarse con un átomo de nitrógeno al que se unen para formar un amino cíclico saturado de 5-a 7- miembros; R4's son iguales o diferentes entre sí y representan halógeno, alquilo inferior; y -OH, -O-alquilo inferior, -NO2, o un grupo representado por la fórmula (II):. n representa O a 2.