Abstract: A controller 3a for a Diesel engine, supplied with source voltage from a DC voltage source (battery) 1 and controlling the current supply to the glow plug 4a via a glow relay 6, includes, in addition to a microcomputer 13 and a reset circuit 14, etc., a source voltage step-up circuit 2. The source voltage step-up circuit 2 includes a source voltage detection circuit, and when the source voltage drops below a predetermined normal level, the circuit steps it up (i.e., raises it) to above the normal level. Thus, even when the engine is started at a low temperature and the source voltage falls below the normal level during the cranking of the engine, the glow plug 4a can be heated to a temperature at which the engine can be started.