PURPOSE: To increase output markedly by arranging a compression-ratio reducing valve for preventing the excessive increase of the pressure in a cylinder, in the combustion cylinder.

CONSTITUTION: A compression-ratio reducing valve 12 is arranged in a combustion cylinder 3. In the early period of compression cycle, the compression-ratio reducing valve 12 is opened, and when a piston 4 comes close to the top dead center at the proper position, the valve is closed, and from this time, compression starts, and the compression-ratio is reduced. In expansion cycle, the compressuion-raion reducing valve 12 is in closed state, and the expansion-ratio can be kept sufficiently large. The timing for the compression reducing valve 12 to be opened is immediately after the start of exhaustion from an exhaust hole 7. Thus, the much amount of gas having the higher pressure than the conventional can be supplied to a gas turbine 2, so the output can be increased exceedingly.