PURPOSE: To easily change displacement with no necessity for changing many parts by changing the displacement by using the same crankcase to change a piston stroke.

CONSTITUTION: When assumed H for compression height of a piston 3, L for length of a connecting rod 4, R for rotational radius of a crankshaft 5 and B for height of a crankcase 2 from its crankshaft center to upper surface, in the case of changing displacement by using the same crankcase 2, the rotational radius R of the crankshaft 5 is changed and also setting the parameters so as to obtain a relation where B=Ra+La+Ha=Rb+Lb+Hb. Thus by changing a stroke, the displacement is changed. Or the length L of the connecting rod 4 is not changed, and a relation between the rotational radius R of the crankshaft 5 and the compression height H of the piston is constituted so as to obtain a relation where Ha+Ra=Hb+Rb.