PURPOSE: To avoid reverse flow of a high temperature combustion gas by leading nitrogen based on the alteration ratio by the pressure difference between pressures of a pressure reformer burner in an anode outlet line or determination of the possibility of reverse flow from the reformer according to the plant operational conditions.

CONSTITUTION: A means 13 to lead nitrogen 103 is installed in an anode outlet line and the nitrogen 103 leading position is made to locate at the point in a reformer burner line side rather than the anode recycle branching point and flowing nitrogen is led to flow to the reformer burner 2a side. When the pressure difference between the pressure in the anode outlet side line and the pressure of the reformer burner 2a becomes less than a prescribed level or the anode outlet side becomes negative pressure, a controlling apparatus 5 sends an opening signal to a nitrogen flowing-in valve 13 immediately and the nitrogen flowing-in valve 13 immediately opens to lead nitrogen. Consequently, nitrogen 103 flows directly to the reformer burner 2a direction and the anode recycle blower 3a and reverse flow of a combustion gas is instantly prevented.