crank angle sensor disposed on an engine. The crank angle sensor measures crank angles through the measurement of angular positions of a gear or a disk attached to the crankshaft or the like at angular intervals of several degrees, and gives signals representing the angular positions to an operating mode detecting unit for detecting an operating mode of the engine. The pressure Pc in the combustion chamber of the engine is substantially equal to the atmospheric pressure Pa at a moment immediately after the exhaust valve has been closed as shown in FIG. 5. If the intake valve is not opened while the piston is moving downward after the exhaust valve has been closed, the pressure Pc in the combustion chamber decreases as indicated by the curve 45 as mentioned above. The pressure Pc in the combustion chamber is given by an expression: