PURPOSE: To reduce both the running cost and equipment cost by using the combustion exhaust gas from a fuel modifier during operation as a supply gas to a vessel of fuel cell, and changing over to inert gas, which is specified as regular material, in case the residual oxygen amount in the exhaust gas exceeds its specified value.

CONSTITUTION: During normal operation the combustion exhaust gas from a fuel modifier 2 is used supplying through a line 28 branched at a combustion exhaust gas line 11, to a vessel 5 of fuel cell via a shutoff valve 29, flow resistant body 30, and vessel gas supply line 32. At a standstill of plant or at the time of no load or low load operation, when a modifier exhaust gas of low residual O2 content is not obtained, an inert gas supply line 23 supplies an inert gas such as N2, CO2 to the vessel gas supply line 32 via a line 26 including a shutoff valve 24, flow resistant body 25, and check valve 27.