A method of operating a fuel cell cogeneration system, the fuel cell cogeneration system comprising: a fuel cell connected to a fuel processing means and having a cooling water pathway; a first heat exchanging means provided on the cooling water pathway; a stored hot water pathway through which stored hot water undergoes heat exchange with cooling water via the first heat exchanging means; a stored hot water tank storing the stored hot water; a second heat exchanging means, which is connected to the stored hot water pathway upstream of the first heat exchanging means, exchanging heat with an exhaust gas of the fuel processing means, a stored hot water flow rate adjusting means, adjusting the flow rate of the stored hot water flowing through the stored hot water pathway; characterized by a stored hot water temperature measuring means, measuring the temperature of the stored hot water flowing into the first heat exchanging means; a flow rate controlling means, controlling the stored hot water flow rate adjusting means on the basis of the temperature of the stored hot water measured by the stored hot water temperature measuring means; and the method comprising the steps of: a step of circulating the stored hot water so as to transfer the heat of the hot water stored in the stored hot water tank to the cooling water pathway of the fuel cell via the stored hot water pathway and the first heat exchanging means during the starting of the fuel cell; a step of measuring the temperature of the stored hot water flowing into the first heat exchanging means; a step of transferring the heat of the exhaust gas from the fuel processing means to the stored hot water via the second heat exchanging means connected to the stored hot water pathway upstream of the first heat exchanging means; and a step of reducing the flow rate of the stored hot water flowing through the stored hot water pathway when the temperature of the stored hot water is a first predetermined temperature or less during the starting of the fuel cell. a fuel cell connected to a fuel processing means and having a cooling water pathway; a first heat exchanging means provided on the cooling water pathway; a stored hot water pathway through which stored hot water undergoes heat exchange with cooling water via the first heat exchanging means; a stored hot water tank storing the stored hot water; a second heat exchanging means, which is connected to the stored hot water pathway upstream of the first heat exchanging means, exchanging heat with an exhaust gas of the fuel processing means, a stored hot water flow rate adjusting means, adjusting the flow rate of the stored hot water flowing through the stored hot water pathway; characterized by a stored hot water temperature measuring means, measuring the temperature of the stored hot water flowing into the first heat exchanging means; a flow rate controlling means, controlling the stored hot water flow rate adjusting means on the basis of the temperature of the stored hot water measured by the stored hot water temperature measuring means; and the method comprising the steps of: a step of circulating the stored hot water so as to transfer the heat of the hot water stored in the stored hot water tank to the cooling water pathway of the fuel cell via the stored hot water pathway and the first heat exchanging means during the starting of the fuel cell; a step of measuring the temperature of the stored hot water flowing into the first heat exchanging means; a step of transferring the heat of the exhaust gas from the fuel processing means to the stored hot water via the second heat exchanging means connected to the stored hot water pathway upstream of the first heat exchanging means; and a step of reducing the flow rate of the stored hot water flowing through the stored hot water pathway when the temperature of the stored hot water is a first predetermined temperature or less during the starting of the fuel cell. a step of circulating the stored hot water so as to transfer the heat of the hot water stored in the stored hot water tank to the cooling water pathway of the fuel cell via the stored hot water pathway and the first heat exchanging means during the starting of the fuel cell; a step of measuring the temperature of the stored hot water flowing into the first heat exchanging means; a step of transferring the heat of the exhaust gas from the fuel processing means to the stored hot water via the second heat exchanging means connected to the stored hot water pathway upstream of the first heat exchanging means; and a step of reducing the flow rate of the stored hot water flowing through the stored hot water pathway when the temperature of the stored hot water is a first predetermined temperature or less during the starting of the fuel cell.