PURPOSE: To prevent acting of undue frictional force to the piston skirt portions, by disposing connecting rods, each having slide grooves on opposite slides of respective turning shafts, at the positions located downwards of cylinders, and fitting the lower ends of piston rods in these slide grooves loosely in a freely slidable manner.

CONSTITUTION: When a piston 5 is moved downward at the time of explosion stroke, its piston rod 9 is also moved downward and a slide pin 13 pushes a slide arm A downward while sliding in a slide groove (a), so that a turning shaft 21 is made to turn. By this rotation of the turning shaft 21, a piston 6 executes its exhaust, suction and compression strokes by the aid of a slide arm B while pistons 7, 8 similarly execute the above strokes respectively by the aid of slide arms C, D and parallelogrammic links 25W28. Further, rotation of the turning shaft 21 drives a crank 35 via a disk 29, a connecting rod 31, a slide unit 33 and a connecting rod 34 and produces driving force. Thus, since the pistons exert the pushing force only in the vertical direction, undue frictional force is prevented from acting to the piston skirt portions, so that vibration of the engine can be reduced.