To provide a leak detection method capable of detecting the amount of a fluid (incompressible fluid) leaked from a sealed part of a piston.

An incompressible fluid such as a liquid is filled in a cylinder 1. The piston 2 is moved to a desired position (a testing position) in the cylinder 1, and a valve 12 on the downstream side of the cylinder 1 is closed. With the valve 12 closed, pressure (testing pressure) is applied to the piston 2 in the direction of an outflow opening 1f. Then if the sealed part (U-rings 2-1 and 2-2) of the piton 2 leaks, the piston 2 moves in the direction of the outflow opening 1f. The amount of the incompressible fluid leaked from the sealed part is detected on the basis of the amount of movement of the piston 2.