A fuel cell system of the present invention includes: a fuel cell (1) supplied with fuel gas and oxidizing gas to generate electricity; a fuel gas supply unit supplying the fuel gas to the fuel cell (1); an oxidizing gas supply unit supplying the oxidizing gas to the fuel cell (1); an aftercooler (7) cooling the oxidizing gas supplied to the fuel cell (1) by heat exchange with a coolant; an oxidizing gas temperature detector (16, 17) detecting temperature of the oxidizing gas; and a coolant circulation controller (21a) starting circulation of the coolant when the detected temperature of the oxidizing gas exceeds a predetermined value. The predetermined value is set to a value of not higher than a minimum electricity generation temperature of the fuel cell (1), and a circulation timing and flow rate of the coolant for the aftercooler (7) are controlled such that the supplied oxidizing gas does not become cold. This enables the fuel cell (1) to generate electricity at cold start-up.