A filter CF1 for filtering dialysis fluid comprises the following: a housing 52 inside of which a filter material 51 is accommodated and which is vertically long； an introduction port 53 that is disposed in the lower part of the housing and that introduces dialysis fluid from an upstream-side flow path 23a； a filtered fluid lead-out port 54 through which filtered dialysis fluid, which has passed through the filter material, is lead out from the upper part of the housing to a downstream-side flow path 23b； and an unfiltered fluid lead-out port 55 through which unfiltered fluid is lead out from the upper part of the housing to a waste fluid flow path 57. When removing the filter from a dialysis fluid circuit 4, fluid is suctioned from the inside of the housing via the upstream-side flow path by using fluid suction means 64A, and a gas is caused to flow into the housing from at least either one of the waste fluid flow path and the downstream-side flow path by using gas inflow means 65. The fluid may be suctioned from the downstream-side flow path and the gas may be caused to flow in from the upstream-side flow path, or the fluid may be suctioned from the waste fluid flow path and the gas may be caused to flow in from the downstream-side flow path. Residual fluid in the filter can be reduced.
