PURPOSE: To enhance a cleaning capability, to avoid damage to a filter medium and to prevent crushing of foreign matter particles with a device for separating and removing the foreign matter, such as algae, sticking to the filter medium by relatively moving the filter medium and a high-pressure nozzle for injecting liquid.

CONSTITUTION: The liquid is injected from the high-pressure nozzle toward the filter medium 8 to separate the solids sticking to the filter medium 8. The separated solids are sucked with a nozzles in a low-pressure or atmosphere pressure and are delivered to a discharge port 6. The entire surface of the filter medium is successively washed by relatively moving the respective nozzles and the filter medium.