A control circuit for a glow plug assembly serving as an engine preheating apparatus is designed so as to effect a temperature control in conformity with the specified position established by selective operation of a key switch having three switching positions, i.e., OFF, ON and START positions. The control circuit comprises a first switch connected between the glow plug assembly and a power source, and a series circuit comprising a second switch connected parallel with the first switch and a resistor. The control circuit is capable of effecting three modes of operation: One is that, when the second switch is switched from OFF position to ON position, the first switch is turned on to rapidly effect a preheating of the engine, and is turned off to interrupt the preheating action when the temperature of the glow plug assembly reaches a predetermined value: Second is that, when a key switch is shifted to the START position, the second switch is turned on and the first switch is intermittently turned on to effect a heat retaining operation so that the temperature suitable for starting the engine is maintained; Third is that, when the key switch is switched from START position to ON position, the first switch is turned off while solely the second switch is turned on thereby to maintain the glow plug assembly at a low temperature.