PURPOSE:To enhance reliability and accuracy of a sensor switch, by connecting the sensor switch and an oil level detecting circuit to an engine igniting circuit, and short-circuiting the voltage produced in a primary winding continously until an engine is stopped. CONSTITUTION:When it is detected by a float 23 that the amount of lubricating oil is reduced to a level lower than a prescribed value, normally open contacts 22a, 22b are closed, so that positive voltage induced by a primary coil 2a in the direction of an arrow A is short-circuited. Resultantly, a capacitor 15 is once charged and then it begins to discharge throuh a closed circuit consisting of the capacitor 15, resistor 13, gate and cathode of a thyristor 10, and resistors 11, 14. Here, if the time constant of the above closed circuit is selected at a value longer than the time of the engine speed at the time of starting engine, negative voltage induced in the primary winding 2a is short-circuited through a series circuit of te thyristor 10 and resistor 11. Thus, it is enabled to lower the high voltage induced in a secondary winding 2b and to stop the engine.