Abstract: A glow plug electrically heated during start-up to help bring the air-fuel mixture of an internal combustion engine to ignition temperature when the engine is cold, instead of being turned off as soon as the engine is running, has its heating current controlled so as to maintain the glow plug temperature within a desired range, as the result of which the engine can be made to run more quietly and more smoothly under many conditions. The current control unit which varies the glow plug current responds to the output signal of a computing unit responsive to various engine operation parameters, including especially a signal representative of the rate of feeding fuel to the engine.