Method of operating fuel cell system

A method of operating a fuel cell system including

• a fuel cell stack,
• a reactant gas supply channel configured to supply a reactant gas into the fuel cell stack,
• a reactant gas circulation channel configured to circulate a reactant off-gas discharged from the fuel cell stack back into the reactant gas supply channel,
• a circulating pump provided on the reactant gas circulation channel and driven by a motor having no rotation detecting sensor, and
• a control unit configured to control rotation of the motor,
• the method comprising:
• a first step of causing the control unit to rotate the motor when starting, and stop supply of electric power to the motor after a rotational speed of the motor has reached a given value;
• and
• a second step of, in an inertial period in which the motor rotates by inertia after the first step, causing the control unit to determine whether or not the rotational speed of the motor has become equal to or lower than a predetermined value within a given time period,
• the control unit being configured to determine, in the second step, that the circulating pump is abnormal if the rotational speed has become equal to or lower than the predetermined value.