PURPOSE: To enable to perform an automatic, reliable, and rapid measurement, by a method wherein, in the measurement of a suction return stroke of a delivery valve, a valve-opening signal is detected photoelectrically.

CONSTITUTION: A delivery valve 1 is secured to an inspecting holder 11 in a condition to push it into a valve seat 2. A displacement detector 17 is a position-electric signal converter such as differential transformers, and has a probing contact 17 at the forward end thereof. A 17a makes contact with the heat part of the valve, and simultaneously, is pressed through the force of a spring 17b. Inspecting oil is fed from a hydraulic source 19 via a circuit 14, but oil is prevented from leaking under a condition that the valve is closed. If the valve is gradually pressed from a lower end by a push rod 13, the valve moves against the spring 17b as it presses the probing contact. The stroke is converted by the detector 17, and is inputted to a displacement to a displacement indicator 18. When a piston part 5 parts from a valve seat 3, an oil current is created in a rotameter 15, and the indicator 18 is held by the passage of a float 15a through a photoelectric detector 16.