To provide a fuel changing method and a fuel changing device for suppressing to the minimum the output fluctuation of the fuel cell, even if the fuel is changed.

The fuel changing device of a fuel cell, which generates electricity using as a fuel the spare fuel and the reformed fuel reformed from the main fuel through a reformer 7, comprises a spare fuel control valve 5 that is installed at the fuel supply passage 4 for supplying the spare fuel to the fuel cell 1, a main fuel control valve 8 that is installed at the supply passage 6 for supplying the reformed fuel reformed from the main fuel through a reformer 7 to the fuel cell 1, and a fuzzy control part 9 that controls the spare fuel control valve at the start of running, so as to supply the spare fuel to the fuel cell, and controls the spare fuel control valve and the main fuel control valve when the temperature of the reformer has risen and reached the normal state, so as to change gradually from the spare fuel to the reformed fuel, and to reduce the fluctuation of output of the fuel cell.