PURPOSE: To clean the filter without detaching it by discharging the water in a water injection nozzle by the pressure generated when the water is heated by a heater and bubbled.

CONSTITUTION: When a filter 7 is increasingly contaminated, a signal is transmitted to a water feed pump 13 from a microcomputer having received a detection signal from a contamination sensor 19, the pump is started to suck up the water in an auxiliary tank 9, and the water begins to discharge from the tip of a water injection nozzle 15. A heater 16 is then energized by the microcomputer, hence the water in the nozzle 15 is heated, and the water in the nozzle 15 is instantaneously bubbled. Consequently, the water in the nozzle 15 is instantaneously increased in pressure, and the water in the nozzle 15 forms a waterdrop bullet by the pressure which is discharged against the filter 7 at a high speed. The dust stuck to the filter 7 is removed by the impact of the bullet, dropped into a drain pan 17 and discharged outside along with the drain water.