PURPOSE: To wash and classify an old filter medium to reuse it by installing a jet pump constituted so that the center axes of a nozzle part and a negative pressure forming pipe may coincide with each other and a filter medium classifying means for classifying filter mediums by diameter.

CONSTITUTION: A jet pump 1 is constituted of a jet opening 15 of a nozzle part 14 in a negative pressure forming pipe 1A so that the center axes of the nozzle part 14, a discharge pipe 18 of a large diameter bend 12 and a small diameter pipe 11 may coincide with each other. Therefore, the jet pump is driven by superhigh pressure and sucks an old filter medium (a) in a filter column X is sucked in a short mime. The filter medium (a) crashes into jet water W to remove and wash the attached dirt, etc. In a filter medium classifying means 3, a fine mesh screen 31 A and several kinds of coarse mesh screens 32A, 33A, 34A of different mesh size are arranged in the opening part of the upper surface of plural hoppers 31, 32, 33, 34 in the order of increasing mesh size from the upper to lower stage and a hopper 35 is arranged for housing the coarse filter medium (a) of maximum diameter at the lowest stage. The filter medium (a) washed and classified is returned to the filter column X again.