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.