To complete inspections on the calibration of the quantity of flow of a flowmeter to be tested having a plurality of types of flow measuring ranges and measurement accuracy by one measuring operation of one piston prover.

Measuring sections A-A1, B-B1, and A-B1 are defined in a cylinder 1 according to flow measuring ranges (small, medium, and large) of the flowmeter 10 to be calibrated. The piston 2 is moved from a start position ST to an end position END. Meanwhile, correction information on the small range is determined on the basis of the difference between the quantity of flow of a fluid having a standard volume and discharged in A-A1 and a measured quantity of flow, and correction information on the medium range is determined on the basis of the difference between the quantity of flow of the fluid having the standard volume and discharged in B-B1 and a measured quantity of flow to set both correction information at the flowmeter 10 to be calibrated. Then the direction of flow is changed over to move the piston 2 from the end position END to the start position ST. Meanwhile, correction information on the large range is determined on the basis of the difference between the quantity of flow of the fluid having the standard volume and discharged in A-B1 and a measured quantity of flow and set at the flowmeter 10 to be calibrated. It is possible to similarly calibrate the quantity of flow on a plurality of serially connected flowmeters to be tested having different measuring ranges of the quantity of flow.