PURPOSE: To measure the instrumental error of a positive displacement flowme ter which varies in discharge quantity periodically with high precision by detecting the position of a piston which moves in a prober.

CONSTITUTION: When a pump 8 is put in operation to start supplying fluid, a flowmeter M to be detected outputs a pulse to a controller 7 at every time its output shaft rotates by a specific angle. A fluid supplied from an auxiliary pipe 4 through the flowmeter to be detected, on the other hand, moves the piston 3 and a scale member 1 to left. A position sensor 5 reads a magnetic scale 1a and outputs a signal to the controller 7. When the flow rate signal from the flowmeter M to be detected is an integral multiple of one period which is set previously and the piston 3 moves almost to an end pat of a cylinder 2, the controller 7 stops the counting operation to close three-way valves V1 and V2; and a flow rate is calculated from the counted at the same time and a flow rate is calculated from a signal counted value from the position sensor 5, so that the instrumental error is calculated from both flow rate values and displayed on a display meter 9.