PURPOSE: To reduce vibration as well as to make heat efficiency improvable while eliminating the reciprocating motion of the piston by installing a piston being clamped to a drive shaft and rotating with it integrally.

CONSTITUTION: When a piston 2 in a combustion mechanism rotates each position of air inlet ports 16, 17 while exhausting a second fire chamber 12 from an exhaust port 13, an on-off valve 11 and an exhaust valve 9 in a first fire chamber 8 are closed, and a fuel mixture pressure inlet valve 10 is opened and thereby fuel mixture is pressed in the first fire chamber 8 from a compression mechanism. When the piston is further rotated, the fuel mixture pressure inlet valve 10 is closed whereby fuel mixture press-in is finished. And, simultaneously with the on-off valve 11 opened, the fuel mixture is ignited and it is made into combustion over the second fire chamber 12 from the first fire chamber 8. With rotation of the piston 2 with this combustion, an exhaust valve 9 is opened and exhaust of the first fire chamber 8, where the on-off valve 11 is closed, is carried out. Thus, the piston 2 integrated with a drive shaft 1 is rotated together with a block cylinder 4, whereby motive powder is securable while reducing vibration and increasing heat efficiency.