A fuel cell system comprising: a first line configured to pass through a fuel cell stack and allow a coolant to circulate therein; a pump provided in the first line; a second line having one end connected to the first line at a first point positioned between an outlet of the pump and the fuel cell stack, and the other end connected to the first line at a second point positioned between an inlet of the pump and the fuel cell stack; a heater provided in the second line to heat the coolant flowing along the second line; and a third line configured to pass through an air conditioning unit, connected to the first line between the first point and an outlet of the fuel cell stack, and configured to allow a part of the coolant to circulate therein.