PURPOSE: To measure frictional force in operation precisely and easily by projecting an upper end part of a cylinder from its inside wall to the center part, forming a hole part smaller than the inner diameter of the cylinder and arranging a gas pressure balancing plate on the upper part of the cylinder and a cylinder block.

CONSTITUTION: When an engine starts, a piston 5 is vertically reciprocated and frictional force is generated between a piston ring and the inside wall of the cylinder 2. The frictional force is measured by detecting the relative displacement of the cylinder 2 to the cylinder block 3. In the combustion process of the engine, gas pressure in the cylinder 2 is balanced and offset by the upward force FU apllied to the upper part 12 of the cylinder 2 and downward force FD applied to the upper surface of the upper end part 12 of the cylinder 2 and the gas pressure balancing plate 15. Consequently, the cylinder 2 is not displaced vertically, so that only the frictional force can be detected.