PURPOSE: To restrain the deterioration of filtration velocity of filter by providing a scraping member on the inner periphery of a cylindrical vessel and scraping off the cake stuck to the outer periphery of a cylindrical filter by the scraping member.

CONSTITUTION: Liquid to be treated is supplied to the peripheral clearance between the cylindrical vessel 12 and the cylindrical filter 11 concentrically installed in the vessel 12 to flow on the outer periphery of the cylindrical filter 11 to carry out the filtration treatment. In this case, the scraping member 18 which is stretched to the longitudinal direction of the vessel and brought into contact with the outer periphery of the cylindrical filter 11 is provided on the inner periphery of the cylindrical vessel 12, and the filtration treatment is carried out during the cylindrical vessel 12 and the cylindrical filter 11 are relatively rotated. As a result, the formation of cake layer on the filter surface is prevented. Consequently, the flow velocity of the liquid to be treated is enough to be such a velocity that the separated solids from the liquid are pushed and flowed through the peripheral clearance, which is very small flow velocity of 1/10W1/100 compared with the ordinary flew velocity of cross-flow filtration.