To make a roller circulate a ring-shaped roller passage by providing a roller bearing without any roller retainer between a pillow and a connecting rod arm.

A semi-cylindrical pillow 3 is abutted on the flat under surface of a piston crown 2a. A pair of right and left arms 44 having a circular arc cross section and extending in the axial direction of a crank shaft is provided on the smaller end of the connecting rod 6. A seat cylinder 5 is supported in the inner hollow part of a skirt 11 and the cylindrically protruding part of the under surface 44b of the arm 44 is supported in the cylindrically recessed part 5a on the upper surface of the seat cylinder 5 in an oscilatatory manner. A roller bearing 61 is intervened between the upper surface 44a of the arm 44 and the under surface of the pillow 5. It is composed of a roller passage 65 provided so as to cross the pillow 3 of the roller bearing 61, a guide plate 64 of a circular arc cross section sandwiched between the cylindrically recessed part 5a on the upper surface 44a of the arm 44 and the cylindrically protruding part on the under surface 3b of the pillow 3, a roller passage 66 composed of an opening 68 provided on the guide plate 64 so as to communicate with the roller passage 65 of the pillow 3, and numbers of rollers 62 circulating between the roller passage 65 and the roller passage. 66.