The Furuhama's movable liner method (1)*, (2) for measuring piston friction force under firing condition has been improved. The new device can avoid the influence of gas pressure of movable liner and also the lateral deformation from the piston slap impulse force.The modified measurement device is illustrated, and then effect by test conditions - cylinder wall temperature, engine speed, and load - on piston friction force is evaluated resulting in similar conclusions to those which obtained by Furuhama et al. (3).Changing the number of piston rings, specifications of both TOP and OIL rings are discussed in terms of decreasing friction force. In the case of TOP ring, the narrower the ring width, the lower the friction force. As for its surface treatment, the gas nitriding treatment was most favorable.As for OIL ring, its tension gives more effect on friction force than its sliding width does.