PURPOSE: To simplify the assembling and reduce the production cost by installing a partition, having a hole capable of piercing a crankshaft, in the inner part of a single cylinder block and forming two cylinder chambers at both sides.

CONSTITUTION: A horizontal groove, opening to an outer circumferential surface at the side where blades 12, 20 are set up, is installed in the middle stage of a single cylinder block 50, and a partition plate 52 is inserted into this groove. Each cylinder chambers 22, 14 is formed in both upper and lower parts of this partition plate 52 which is provided with a hole 53 in which a crankpin 6a or 6b or is pierceable. At time of assembling, after the partition plate 52 is inserted into the groove, a crankshaft 6 is pierced into the hole 53 of the partition plate 52, and the blade 20 is assembled so as to make the lower end come into contact with top of the partition plate 52, while the blade 12 is also assembled so as to come into contact with the lower end of the parition plate 52. Consequently, it is assembled in the single cylinder block 50 whereby assembling manhour can be reduced while reducing the cost.