To provide a low cost fuel cell cogeneration system capable of suppressing possibility of generating hot water shortage.

The fuel cell cogeneration system includes a fuel cell system 1, a hot water tank 2, an exhaust heat recovery line 22 returning heated water by absorbing exhausted heat from the fuel cell system 1 to the hot water tank 2, a hot water supply piping 23 extending from the hot water tank 2 to outside, a hot water supply shutdown valve 5 arranged on the way of the hot water supply piping 23, an auxiliary hot water tank 4, an auxiliary pressurizing piping 25 extending from the hot water tank 2 to the auxiliary hot water tank 4, a heater 8 heating water in the auxiliary hot water tank 4, an auxiliary piping 24 extending from the auxiliary hot water tank 4 to a downstream side of the hot water shutdown valve 5 of the hot water supply piping 23, an auxiliary shutdown valve 6 provided in the middle of the auxiliary piping 24, a hot water supply thermometer 3 measuring temperature of water sent out from the hot water tank 2 to the hot water piping 23, and a control device 9 which closes the hot water shutdown valve 5 and opens the auxiliary shutdown valve 6 when the transmitted temperature measured by the hot water supply thermometer 3 becomes below the predetermined detection temperature with which it is impossible to supply hot water.