To provide a sewage treatment apparatus constituted so as to easily keep the filtering function of a carrier filter bed in a carrier treatment tank by backward washing while enhancing the function.

This sewage treatment apparatus is equipped with the carrier treatment tank E2 which is provided with the carrier filter beds E21 and E22 respectively formed by laminating a large number of carriers C1 and C2. A large number of the first carriers C1 comprising a material whose specific gravity is lower than that of water are floated and laminated in the carrier treatment tank E2 (E) to form the carrier filter bed E21 and a large number of the second carriers C2 comprising a material whose specific gravity is higher than that of water are deposited and laminated in the carrier treatment tank E (E2) to form the second carrier filter bed E22.