Method of operating a fuel cell apparatus in which a reforming reaction in the reforming portion is selected by a controller at the starting time of the apparatus by comparing a first starting temperature of a reforming portion to a temperature T1 at which steam reforming can be performed and comparing a second starting temperature of a vaporizing portion to a temperature T2 at which a predetermined amount of steam can be generated by steam reforming. A reforming reaction starting with an autothermal reforming reaction is performed when the first starting temperature is not lower than T1 and the second starting temperature is lower than T2.