To provide a power source device capable of being miniaturized and a power source control method, with respect to the power source device and the power source control method comprising a MOS transistor mounted between an input terminal and an output terminal, and controlling electric current supplied from the input terminal to the output terminal, and a voltage control means detecting the voltage of the output terminal and controlling the MOS transistor to make the voltage of the output terminal constant.

In this power source device having the MOS transistor M1 mounted between the input terminal (Tin) and the output terminal (Tout) and controlling the electric current supplied from the input terminal (Tin) to the output terminal (Tout), and the voltage control means R1, R2, 131, 132, 133 detecting the voltage of the output terminal (Tout) and controlling the MOS transistor M1 to make the voltage of the output terminal (Tout) constant, backward current preventing means 231, 232, 233 are mounted to switch off the MOS transistor M1 when a state between the input terminal (Tin) and the output terminal (Tout) is a reverse bias state, to prevent backward current.