A seal is brought into contact with a filter media, a filter pump is operated to start filtration, and a signal from a pressure sensor is received. When a predetermined value is exceeded by the signal from the pressure sensor, the filter pump is stopped, the seal is separated from the filter media, and take-up of one section of the filter by a filter collection roll is started. In finishing filter supply from a filter supply roll, drive of a motor is stopped, and an alarm signal is transmitted. If the supply is not to be finished, one section of the filter is taken up by the filter collection roll, and the next stage of filtration is entered.