In a system comprising a fuel and a rotating electrical machine, damage of a switching element is prevented when the rotating electrical machine becomes a locked state. In a power controller, it is monitored whether the rotating electrical machine is in the locked state or not. When the rotating electrical machines is judged to be in the locked state, a command for dropping output voltage is given to the fuel cell. Thus, inverter input voltage can be dropped, loss power of the switching element in the rotating electrical machine is dropped and damage can be suppressed. The rotating electrical machine is monitored to cancel the locked state or not while dropping of inverter input voltage is controlled. When the locked state is judged to be canceled, control of the fuel cell is returned to a state of regular operation control.