PROBLEM TO BE SOLVED: To provide a solid oxide fuel battery system so that the cracking of a fuel battery cell due to reduction expansion of an oxygen electrode can be suppressed by controlling an oxygen partial pressure inside a battery housing.SOLUTION: A solid oxide fuel battery system comprises: a fuel battery cell 8 having a solid electrolytic film 10, a fuel electrode 12 provided on one side of the solid electrolytic film 10, and an oxygen electrode 14 provided on the other side of the solid electrolytic film 10; a quality-modification device operable to modify fuel gas in quality; and a battery housing 54 in which the fuel battery cell 8 and the quality-modification device are housed. In the solid oxide fuel battery system, the modified fuel gas resulting from quality modification by the quality-modification device is supplied to the fuel electrode 12, and oxidation gas is supplied to the oxygen electrode 14. The solid oxide fuel battery system further comprises an oxygen nonstoichiometric material 74
