To provide an efficient and inexpensive filtering medium washing method, in a filtration device used for removing a suspended matter in a fluid.

This filtration method/device is constituted to include a filter side face, and a nozzle in a lower part of a filtration layer surface layer face, with structure to generate a turning flow in a filter inside. Efficient washing is carried out by making a gas-liquid mixture fluid flow in from the nozzle just before backwashing a filter, and by executing conventional backwashing after separating the suspended matter accumulated a lot in the vicinity of a filtration layer surface. No special device such as a cylinder and a compressor is used for supplying gas to the nozzle, but by supplying the gas by use of natural intake, a cost is reduced thereby.