A filter device has a filter housing, which is intended to contain a liquid to be filtered and in which at least one filter rod is arranged. The filter rod is composed of annular washers, which are arranged in a stack on a core formed with openings and which are so closely packed that particles in the liquid adhere to the periphery of the washers when the liquid is caused, from the outside, to pass between the washers into the core for filtration. The filter device has a gas supply member, which is intended to inject a gas flow into the liquid in the filter housing.In a method of cleaning a filter rod for filtering a particle-containing liquid in such a filter device, a gas flow is injected into the liquid in the filter housing so that a particle sock, which is formed on said filter rod of particles in the liquid, is detached from the filter rod and disintegrated.