Abstract: A filter medium is backwashed to cleanse it of cake when a fluid pressure difference across the filter medium rises to a predetermined degree. Since the pressure difference is subject to change not only with the amount of the cake but also with the temperature and flow rate of the fluid being filtered, not only the pressure difference but also at least either of the fluid temperature and flow rate is constantly monitored. A correction coefficient circuit is provided which has stored thereon pressure difference characteristics at various temperatures and/or flow rates of the fluid including a pressure difference at a rated temperature and/or rated flow rate. The correction coefficient circuit computes a correction coefficient by comparing the pressure difference characteristic at the monitored temperature and/or monitored flow rate of the fluid with the pressure difference characteristic at the rated temperature and/or rated flow rate of the fluid.