A fuel cell system (100) includes: a fuel cell (12) configured to generate electric power by causing a fuel gas and an oxidizing gas to react with each other; a casing (11) accommodating at least the fuel cell (12), the casing (11) including an air inlet (16) and an exhaust outlet (17) formed therein; a supply passage (18) connected to the air inlet (16), the supply passage (18) being configured to introduce external air into the casing from outside of the casing; an exhaust passage (19) connected to the exhaust outlet (16), the exhaust passage (19) being configured to exchange heat with the supply passage (18) and discharge at least air inside the casing to the outside of the casing; an air supply device (15) configured to introduce the external air into the casing through the supply passage (18); a temperature detector (20) disposed such that at least one of the inside of the casing (11), the supply passage (18), and the exhaust passage (19) is provided with the temperature detector (20), the temperature detector (20) being configured to detect a temperature; and a controller (21) configured to control at least the air supply device (15). If the temperature detected by the temperature detector (20) after the controller (21) has caused the air supply device (15) to operate is lower than or equal to a first predetermined temperature, the controller (21) reduces an amount of air supplied by the air supply device (15) and causes the air supply device (15) to continue operating.