Provided is a calibration method for flowmeters in a blood dialysis system, whereby highly accurate calibration can be performed even by omitting a reference flowmeter not directly relating to dialysis therapy. This calibration method comprises: while preventing outflow into a blood channel in a blood purifier, supplying a liquid to a channel, said channel passing through an inflow flowmeter and an outflow flowmeter; and then calibrating the inflow flowmeter and the outflow flowmeter correction whereby, at the time of liquid supply, a value measured by the outflow flowmeter is equalized to a value measured by inflow flowmeter.