To prevent generation of large vibration in an engine so that its assembly work can be easily performed, even in the case of improving an output of the engine.

Five or less cylinders 7 are mountably/demountably tightened to a crankcase 2 supporting a crankshaft 4, an output shaft 37 transmitting power of the crankshaft 4 to a flywheel 42 is provided. The cylinder 7 is constituted by a cylinder main unit 9 having a cylinder hole 8 and a cylinder head 10 mounted in this cylinder main unit 9. An axial direction one end of the cylinder hole 8 is opened toward inside the crankcase 2, the other end is closed in the cylinder head 10. A reduction gear mechanism 39 is interposed between the crankshaft 14 and the output shaft 37, the cylinder main unit 9 and the cylinder head 10 are integrally molded with each other, a balancer 46 is connected to be associated with the crankshaft 4.