To provide a system and its operation method capable of continuous operation regardless of a thermal load, in a fuel cell exhaust heat utilization system using fuel cell exhaust heat for water heating/space heating and the like.

In the case of T2>42°C, the number of revolutions of a circulation pump P1 is reduced to reduce a flow rate of a primary circuit (S102). Thereby, a heat quantity supplied to a load heat exchanger 5 is reduced, and a heat supply quantity to a floor heating panel 6 being a secondary heat using facility is reduced. In the case of T2≤42°C, whether or not T2 is below 38°C is then determined (S103). In the case of NO, the operation is continued in that state (S105). In the case of YES, it shows that the temperature T4 of a secondary return path is below an allowable temperature range. Thereby, the number of revolutions of the circulation pump P1 is increased to increase the flow rate of the primary circuit (S104), and a heat quantity supplied to the heat exchanger 5 is increased.