To effectively improve energy utilization efficiency of high pressure gas of an air pressure engine, prevent locking of a piston caused by dew condensation of an engine cylinder, improve a shielding effect of an air supply valve, reduce compression resistance force of the cylinder, and improve a power output.

A cylinder cap 5 covers a cylinder body. A back-opening type compressed air supply valve 7, an inlet valve 11, and an exhaust valve 6 are installed in the cylinder cap 5. An air supply cam and an exhaust cam are installed on a camshaft. A rotation of a crankshaft moves the camshaft by a timing chain or a belt. Opening and closing of the inlet valve 11 and the exhaust valve 6 are controlled via a rocker arm. Further, a reciprocating motion of a piston and discharge of used low-pressure air from a cylinder 2 are controlled by supplying high pressure gas into the cylinder 2. A ratio between a bore and a piston stroke of the cylinder 2 of a piston-type air pressure engine is 1:2 to 1:15, optimally, 1:10.