A piston type pneumatic engine, including a cylinder block, a piston, a cylinder, a crankshaft, a connecting rod, a camshaft, and a device for controlling the opening or closing of a gas valve. A cylinder cover arranged on the cylinder block is provided with an inwardly-opened type compressed-gas intake valve and an exhaust valve. An intake cam and an exhaust cam are arranged on the camshaft. The opening or closing of the intake and the exhaust valves is controlled by a rocker arm which is driven by the camshaft. The camshaft is driven by the rotated crankshaft via a timing chain or belt.