A method is provided for detecting filter clogging which are able to keep track of the filter clogging with precision and in detail. At least one pressure selected from the group consisting of a pressure Pa in a blood inflow portion (32a), a pressure Pv in a blood outflow portion (32b), a pressure Pf1 in a filtrate inflow portion (32c), and a pressure Pf2 in a filtrate outflow portion (32d) is measured continuously, then a variation (pressure waveform) of the pressure is analyzed, a clogging of a filter (32) is detected using the analyzed result. Thereby a clogging of a filter is detected early. As a result, it is possible to prevent from developing of the filter clogging by controlling a dosage amount of anti-coagulant properly and changing a flow rate of blood. The variation of the pressure wave may be analyzed by subjecting the variable to Fast Fourier Transform.