NOVELTY - A fuel cell system comprises fuel cell (1) for operating in an operation mode; coolant supply device (15) for circulating an amount of coolant through coolant path for cooling fuel cell; fuel cell temperature detector (20b) for detecting fuel cell temperature; coolant temperature detector (20c) for detecting coolant temperature in coolant path; and controller (30) for controlling the amount of coolant circulated by coolant supply device, selecting operation mode, and calculating difference between detected coolant and fuel cell temperatures.    USE - A fuel cell system for vehicle, e.g. automobile, truck or bus.    ADVANTAGE - The system stops power generation through fuel cell in a low load condition; thus preventing fuel cell from deterioration due to a thermal shock, fuel gas from leaking due to temperature difference, fuel efficiency of fuel cell from decreasing, rotational speed of cooling water pump from varying in short time, and user (i.e., driver of vehicle) from feeling uncomfortable as a result of frequent variation in the pump rotational speed.    DETAILED DESCRIPTION - Fuel cell system comprises fuel cell (1) for operating in an operation mode selected from power generation mode and power generation stop mode; coolant supply device (15) for circulating an amount of coolant through coolant path for cooling fuel cell; fuel cell temperature detector (20b) for detecting fuel cell temperature; coolant temperature detector (20c) for detecting coolant temperature in coolant path; and controller (30) for controlling the amount of coolant circulated by coolant supply device, selecting operation mode of fuel cell between power generation mode and power generation stop mode, and calculating difference between detected coolant and fuel cell temperatures. While the operation mode of fuel cell is selected to be the power generation stop mode, the controller increases the amount of coolant circulated as the difference between detected coolant and fuel cell temperatures increases. An INDEPENDENT CLAIM is included for a method of preventing thermal shock to a fuel cell in fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing is a block diagram of fuel cell system.    Fuel cell (1)    Coolant supply device (15)    Fuel cell temperature detector (20b)    Coolant temperature detector (20c)    Controller (30)