A power supply is provided, which is capable of stabilizing a generated output of the fuel cell using neither a voltage nor a current outputted from the fuel cell as a negative feedback signal. The power supply includes a fuel cell 110, a DC-DC converter 120 that adjusts a voltage outputted from the fuel cell 110 according to a PWM signal and then outputs the voltage to a load device 200, a switching controller 130 that generates the PWM signal and outputs this signal to the DC-DC converter 120, a voltmeter 140 that measures a voltage Vout outputted from the DC-DC converter 120, and a rechargeable battery 150 connected to the load device 200 in parallel. The DC-DC converter 120 calculates a duty ratio of the PWM signal by performing a specific computation using the voltage Vout and a target fuel cell voltage Vt.