Abstract: The blood of a patient to be dialyzed is circulated out of the patient's body through an extracorporeal circuit including a blood pump and a dialyzer. The dialyzer has a solution supply piping for the supply of a dialyzing solution thereto and a discharge piping for the discharge of the dialyzing solution therefrom, and the discharge piping is branched into a discharge passage and a metering passage. The supply piping, the discharge passage and the metering passage have disposed thereon a supply control valve, a discharge control valve and a combination of a metering valve and a metering device, respectively. During a washing step, the supply control valve is opened while the discharge and metering control valves are opened and closed, respectively, so that the dialyzing solution within the dialyzer is replenished.