Abstract: A pre-heating device, for starting an internal combustion engine, of the Diesel type or the like, comprises a pre-heating plug, mounted in each cylinder, adapted to be supplied by a source of electrical energy to ensure the pre-heating of a combustion chamber, and a rapid pre-heating circuit ensuring the connection of each pre-heating plug to a relatively high electrical voltage. The rapid pre-heating circuit is normally put into operation from the start of pre-heating. A slow pre-heating circuit is also provided to ensure the connection of each pre-heating plug to a lower electrical voltage after a rapid pre-heating period determined by, first time delay means. Said first time delay means is combined with means sensitive to the temperature of the engine arranged so that the rapid pre-heating time depends on the temperature of the engine.