Solid oxide fuel cell system responsive to explosive vaporization of water in reformer to control power

To provide a fuel cell system with which degradation of fuel cells can be suppressed by avoiding the ill effects caused by bumping of water used for steam reforming during the electrical generation. The present invention is a solid oxide fuel cell system including: a fuel cell module, a fuel flow regulator unit, a reformer for reforming supplied fuel, a vaporizing section for supplying steam to the reformer, a water flow regulator unit, and a controller for controlling the fuel supply device and water supply device, and for controlling the power extracted from the fuel cell module; whereby the controller is furnished with: a bumping determination circuit for determining the occurrence of excessive vaporization of water in a vaporizing chamber, and an extracted power limiting circuit for limiting the power when the bumping determination circuit has determined excessive bumping of water.