Frictional force as a function of crank angle of a piston assembly and piston rings alone were measured after the following devices were developed. (1) A gas sealing device that did not affect the measuring values. (2) A device to minimize the effect of gas pressure on the cylinder head and block deformations. (3) A device to minimize the effect of piston slap force. From the measurement of the frictional force diagrams the following characteristics have been found. (1) Lubricating oil temperature has the greatest effect upon the frictional loss of the piston. (2) Piston friction does not increase to the point of becoming proportional to the engine speed. (3) Friction in the expansion stroke increases at high load by the piston slap phenomenon. But the increase of total losses are small because the duration is short. (4) Piston rings account for the majority of the entire frictional force of the piston.