PROBLEM TO BE SOLVED: To provide a method of estimating the soundness of a fuel gas passage of a solid oxide type fuel battery.SOLUTION: Methane containing no water vapor is analyzed by a gas chromatograph, methane-contained gas and water vapor are mixed to prepare reaction gas, the reaction gas is supplied to a fuel gas introducing portion of a fuel electrode containing a fuel gas passage of SOFC to exhaust off-gas from a fuel gas exhaust portion of the fuel electrode containing the fuel gas passage, water vapor is removed from the off-gas to prepare analysis off-gas, the component amount of the analysis off-gas is analyzed by the gas chromatograph, the component amount of the reaction gas is calculated from the component amount of the methane-contained gas and the water vapor containing percentage of the reaction gas, mass balance calculation is executed in consideration of a chemical reaction occurring at the fuel electrode containing the fuel gas passage by using the component amount of the reaction gas a
