To provide a temperature control system capable of reducing costs and suppressing temperature error of a heating element.

ECU (electric control unit) 40 opens and closes a power supply passage from a battery 21 to a glow plug 20 by GDU (glow plug driving unit) 30. More concretely, ECU 40 opens and closes GDU 30 by an opening/closing control signal. Here, a temperature of the glow plug 20 is controlled by duty of the opening/closing control signal. In temperature control of the glow plug 20, GDU 30 superposes battery voltage (plug driving voltage) input through a harness 22 to a diagnostic signal. Further, ECU 40 outputs the opening/closing signal of the duty according to the plug driving voltage superposed to the diagnostic signal.