An arithmetic and control unit is configured to calculate a resistance value of a glow plug on the basis of an energization current of the glow plug and a voltage applied to the glow plug, perform a multiplication of the resistance value and a constant that has been determined beforehand on the basis of an electrical characteristic of the glow plug, input a predetermined heater reference point temperature, calculate an offset with a predetermined offset arithmetic expression from the heater reference point temperature, correct the multiplication result with that offset, and take the correction result as an estimated temperature of a tip of the glow plug.