PURPOSE: To facilitate assembly of a snap ring for preventing falling of a piston pin.

CONSTITUTION: A stepped part 8 is formed in the position, having a radius higher than the hole diameter of a piston pin, of the outer end part of a piston pin hole 5. A stepped surface 8a of the stepped part 8 is smoothly connected to an inner peripheral surface 5a of the piston hole through a curved part 9 brought into peripheral contact with the inner peripheral surface 5a of the piston pin hole. Thus, by grasping a part of a snap ring 10 by means of a tool T and rotating it in the direction of an arrow mark, the snap ring 10 is reasonably reduced in a diameter as it is slid over the surfaces of the stepped part 8 and the curved part 9, whereby engagement of the snap pin with a groove part 6 for assembly can be facilitated.