A filter device has a filter tank into which a contaminated fluid is introduced, a magnetic ball filter medium using a large number of metal balls, magnets, and lift rods. The magnetic ball filter medium is formed of an aggregate of a large number of spherical metal balls of a magnetic material and is held in a filter medium storage portion in a manner such that the metal balls are movable relatively to one another. The magnets are movable between a first position and a second position. When in the first position, the magnets apply a magnetic field to the metal balls, thereby causing the metal balls magnetically to attract one another and so that the metal balls are fixed to one another. When in the second position, the magnetic attraction between the metal balls is canceled so that the metal balls are movable.