To provide a washing method of removing stains from the dialysate line in an artificial dialyzer to disinfect securely, while effectively preventing a hollow fiber membrane type dialysate filter located in the dialysate line from being deteriorated and reduced in function.

The dialysate line including the dialysate filter formed by bonding at least one side of a hollow fiber membrane bundle with a polyurethane base resin layer, is washed both with an aqueous hypochlorous acid-free alkaline solution and with an aqueous peracetic acid solution.