To enable piston pressing force to be generated by an explosion of fuel so as to be convertible to the torque of a turning shaft at high efficiency.

This reciprocating internal combustion engine is provided with such a structure that a piston 4 is reciprocably set up in a cylinder 2, while a piston pin 6 of the piston 4 is rotatably attached to one end of a connecting rod 10 via a bearing 8, and a crank pin 14 of a crankshaft 12 is rotatably attached to the other end of the connecting rod 10 via the bearing 8. A turning center C2 of the piston pin 6 passes through a turning center C1 of the crankshaft 12, and it is decentered as far as a distance (d) to the left from a reference surface RS in parallel with a center axis R1 of the cylinder 2. The crankshaft 12 is rotated in the direction of an arrow W so as to make a part to be crossed with the reference surface RS move to the right from the left at a spot nearer to the piston 4 than the turning center C1. In addition, a plurality of lubricating oil flowing holes 5 are made up in a skirt part of the piston 4.