A fuel cell system (100) includes a fuel cell (10), a fuel gas supply/exhaust portion (50, 60), an oxidant gas supply/exhaust portion (30, 40), a cooling portion (70), and a controller (20). The controller performs at least one of a transient increase control process and a transient decrease control process. In the transient increase control process, the controller determines whether a temperature of a coolant is in a transient increase state. In the transient increase state, the controller performs an oxidant gas pressure increase process. In the transient decrease control process, the controller determines whether the temperature of the coolant is in a transient decrease state. In the transient decrease state, the controller performs at least one of the oxidant gas pressure increase process and an output increase process. In the output increase process, the controller controls the fuel cell to generate an output higher than a target output corresponding to a request output.