PURPOSE: To enable to shorten the time required for a glow plug to be heated to a required high temperature after turning an engine switch ON, by starting high-temperature control before turning the engine switch ON.

CONSTITUTION: When a key is turned for starting an engine in the state that a switch 35 is closed, a self-holding switch 21 is closed and current is supplied temporarily to all circuts. Here, if the engine temperature is low to such an extent as to require heating of a glow plug 1 and the engine is at rest, a transistor 27 is turned ON by a bias imparted from a an engine temperature detecting circuit 17 via a resistance 29 and a relay 26 is energized to keep a self- holding switch 21 in its closed state. Here, since a comparison circuit 8 produces a high-level output, an electric contact of a main relay 3 is closed and the glow plug 1 can be heated to a required high temperature within a few seconds although the engine key 21 is not turned ON.