PURPOSE: To follow in response to variation of load by supplying raw fuel to a fuel process unit through a control valve, and installing a fuel tank which stores reformed fuel between the fuel process unit and a fuel cell.

CONSTITUTION: Raw fuel is supplied to a fuel process unit 1 comprising a reformer which reforms steam and a shift converter which converts carbon monoxide to carbon dioxide, through a control valve 2 which is controlled according to the variation of electric output through a control unit 10, and supplied to a fuel cell 5 through a fuel tank 9 which stores reformed gas. Lack of reformed gas caused by sharp increase of electric output is temporarily suplied from the fuel tank 9, and the control valve 2 is opened in advance. Sharp increase of load is followed by instantaneously supplying reformed gas required and responding ability is improved by setting flow rate or raw fuel.