An electric system has a fuel cell and an electric storage device which are connected in parallel to each other with respect to a propulsive motor, and a DC-to-DC converter connected closer to the electric storage device than a junction where the fuel cell and the electric storage device are connected in parallel to each other. An electric power control system includes a power supply controller having a failure detector for detecting a failure of the DC-to-DC converter. When the failure detector detects a failure of the DC-to-DC converter, the response of electric power output from the fuel cell is limited.