A connecting member coupled to four pistons that reciprocate within cylinder bores is coupled to a crankpin of a crankshaft, and a pinion member which integrally rotates with a crankshaft portion is capable of rolling along the inner periphery of an internal gear member and has an outer diameter equal to ½ of an inner diameter of the internal gear member. The crankpin executes a reciprocating rectilinear motion through the rotation and revolution of the pinion member, and a journal support member has a bearing for supporting a crank journal so as to rotate positioned between the pinion member and a crank arm, and which is supported by a case member so as to rotate coaxially with an output member.