With a diesel engine driving an automobile, it is usually necessary to heat up glow plugs to a high temperature of, for example, eight hundred degrees centigrade which is suitable for quickly starting the diesel engine, prior to performing the starting operation of the engine by means of a known starter motor. However, under certain environmental temperature conditions of a diesel engine, for example, under a condition where the diesel engine per se is sufficiently warmed up, it is possible to start the heating of the glow plugs at the same time as operating the starter motor which operation is performed by shifting a key switch to a starting position thereof.
A conventional control apparatus for glow plugs disclosed in, for example, U.S. Pat. No. 3,675,033 belonging to Richard et al does not always take such environmental temperature condition of a diesel engine into consideration for achieving control of the heating of glow plugs.