PURPOSE: To reduce the number of parts and simplify a shape by providing units around a bore on a bottom plate on the lower part of a main frame and fixing said units around the bore to the side wall part of the base plate of said main frame.

CONSTITUTION: A wet finned cylinder liner 12 is temporarily fixed to a side wall part 2 by means of a U-volt 14 and a cylinder head 19 is set after a side plate 17 is installed. A long-size tension bolt 23 is inserted into a positioning hole, and is fastened to the cylinder head 19 with a tapped hole 24. And, the base-plate side wall part 2 and the wet finned cylinder liner 12 are finally fastened and fixed to each other from their temporarily fastened condition, by double nuts 20. The elastic deformation of an O-ring 22 suffices as to the structure of water tightness for a water jacket part 25. Thereby, a structure in an integrated form having a characteristic of good natural frequency can be obtained, reducing the number of parts while simplifying a shape.