A plurality of reference pressures for pressure in an intake passage of the engine is stored in a memory so as to be derived in accordance with speed of the engine and opening degree of a throttle valve of the engine. A reference pressure is derived from the memory in accordance with the engine speed and the corrected opening degree. The pressure in an intake pipe is compared with the reference pressure. A fail-safe signal is generated when the intake pipe pressure is higher than the reference pressure. In response to the fail-safe signal, the fuel supply for the engine is cut off when the engine speed is higher than a predetermined speed.