An initial driving voltage fixing the desired luminance of a light-emitting source is set at a voltage lower than that of a power supply; the light-emitting source is driven by applying a light-emitting source driving voltage at the initial driving voltage; and then, a CPU determines whether or not the power supply voltage has dropped down the initial driving voltage. When the power supply voltage has become lower than the initial driving voltage, the CPU applies the light-emitting source driving voltage to the source at the power supply voltage, and holds the undervoltage information of the power supply that the power supply voltage has dropped. When the CPU determined that the power supply voltage had returned higher than or equal to the initial driving voltage after holding the undervoltage information, the CPU gradually returns the light-emitting source driving voltage to the initial driving voltage.