The disclosed fuel cell system (FCS) is intended to suppress backflow of water from the exhaust line outlet, which emits reaction off-gas, without any decrease in the performance or fuel efficiency of the fuel cell (FC), and the exhaust line is configured such that emission of reaction off-gas can be switched between a main exhaust line (ML) and a sub-exhaust line (SL) by means of a switching valve (SV). The sub-exhaust line (SL) has an ascending sloped section (SL1) which is shaped with a steeper upward slope than the slope of the main exhaust line (ML), and further along in the direction of flow, a descending sloped section (SL3) that is shaped with a downward slope. The switching valve (SV) switches so as to emit via the main exhaust line (ML) when the reaction off-gas emission level meets or exceeds a threshold emission level, and switches so as to emit via the sub-exhaust line (SL) when the reaction off-gas emission level is below the threshold emission level.