A power supply device that switches one of a first power supply, a second power supply, and a third power supply, all of which supply power to an auxiliary device, to a transfer gate in a CMOS image sensor having a photodiode and outputs the corresponding power to the transfer gate is disclosed. The device includes: a first transistor driven by the second power supply and outputting power of the second power supply to the transfer gate; a second transistor driven by the second power supply and outputting power of the first power supply to the transfer gate; a third transistor driven by the third power supply and outputting power of the third power supply to the transfer gate; and a fourth transistor located before the second transistor, driven by the first power supply, and outputting power of the first power supply to a source of the second transistor.