PURPOSE: To easily and securely hold a spherical body by laser-welding a spherical body holding member to a skirt of a piston with force-contact force functioned to the spherical body in an apparatus wherein the spherical body of a connecting rod is force-contacted to a seat of the piston via the spherical body holding member.

CONSTITUTION: A piston device 17 is formed with a ring part 21 to be fitted into a crank pin at an end of a connecting rod 18 and formed with a spherical body 20 on the other end. A skirt 22 is cylindrically extended from an external end toward a shaft on a piston 19 while a seat 23 is formed on the inner face. Further the spherical body 20 is held via a spherical body holding member 24 with respect to the seat 23. In this case the spherical body holding member 24 is inserted into an inner periphery of the skirt 22 as well as an elastic end 24a in a half-doughnut shape is brought into contact with the spherical body 20. With the spherical body 20 in contact with the seat 23 by force for applying a pressure in a direction A, the member 24 is laser-welded to the skirt 22.