An engine comprising a main intake passage having a throttle valve therein. A bypass passage is branched off from the main intake passage located upstream of the throttle valve and is connected to the main intake passage located downstream of the throttle valve. A flow control valve, actuated by a step motor, is arranged in the bypass passage. When the pressure of the lubricating oil of the engine is decreased below a predetermined pressure, the idling speed of the engine is increased to a predetermined speed. In addition, when the temperature of the cooling water of the engine is increased beyond a predetermined temperature, the idling speed of the engine is also increased to a predetermined speed.