A fuel cell system is provided in which the concentration increase of discharged fuel gas is inhibited even when abnormality is detected in a discharge unit. A fuel cell system in which a fuel gas discharged from a discharge unit (SV5) is diluted with an oxidizing gas (14) and discharged comprises an abnormality detection unit (205) for detecting an operation abnormality of the discharge unit (SV5), and a change unit (206, 207) for changing the supplied quantity of the oxidizing gas when an abnormality of the discharge unit (SV5) is detected, and the concentration of the diluted (14) and discharged fuel gas is changed and inhibited by changing the supplied quantity of the oxidizing gas.