PURPOSE:To obtain valve timing of an engine smoothing the action of its valve and enabling its high speed running, by utilizing a cavity in a piston so as to increase a lift of the valve when the piston is positioned in its top dead center. CONSTITUTION:An engine, not having a throttle valve, performs intake and exhaust operation by a single valve 1. In an exhaust stroke, exhaust gas is allowed to flow out from a reed valve 4 via a guide plate. An intake port 2, branching and providing a reed valve 4', never causes an inflow of the exhaust gas. A piston 7 never collides against the valve 1 by providing a cavity 8 in the piston 7 even if a valve lift (h) is in its maximum when the piston is placed in its top dead center at the intake and exhaust operation of the engine. Accordingly, the valve never generates its dancing action at high speed running by forming a cam in a smooth shape.