Power source device for electric motor

The device has accessory circuits (7, 8) connected in parallel to an output terminal (1A) of a fuel cell power plant (2). A secondary battery (3) connected to the terminal performs a charging and discharging operation. A direct current/direct current (DC/DC) converter (6) regulates an input and output voltage of the battery in the discharging operation. The converter is disposed between the battery and terminal.