The power supply apparatus of the present invention comprises a fuel cell, a secondary battery for supplying power to a load, a power measurement section for measuring the output power of the fuel cell, a power converter which inputs output of the fuel cell, controls the output power so that the input voltage or the input current approaches the operation target value, supplies the output power to the load and/or the secondary battery and conducts a power conversion, and a control section which controls quantity of fuel to be supplied to the fuel cell, and also indicates to the power converter of an operation target value that comes within predetermined range which includes the smallest quantity of fuel the fuel cell discharges, wherein the control section sequentially indicates a plurality of the target values for power measurement to the power converter, the power measurement section measures the output power of the fuel cell for each target value for power measurement, and the control section seeks the target value which gives the maximum power within a plurality of output powers, and updates the operation target value based on that value.