PURPOSE: To enable remaining plugs to start an engine even if some glow plugs are disconnected, by detecting resistance of an exothermic coil in the glow plug corresponding to a set temperature to hold temperature of the glow plug at a preheating set temperature.

CONSTITUTION: Assuming that two of four glow plugs 1, for example 1a,1c are disconnected, the composite resistance value is two times that of the four normal glow plugs. Thus, when a key switch 2 is closed, a transistor Tr2 becomes non- conductive and high eary resistance value of the glow plug is detected by a second bridge circuit and a second comparator C2 to operate an amplifying circuit 4 while operating a second relay RL2 through a timer 5. Then, a third relay RL3 is operated to close the normally opened contacts rl3 and insert resistance R4 in parallel into resistance R2 forming a first bridge circuit so that the composite resistance is halved. Thus, even if some blow plugs would be disconnected, temperature of the remaining glow plugs 1 can be maintained at a set temperature.