A fluid circuit system comprising: a heating/cooling apparatus for heating or cooling a fluid; a fluid utilization apparatus (40, 50a-50c) that is connected to the heating/cooling apparatus by a pipe and adapted to utilize the heated or cooled fluid and a flow rate control device adapted to control a flow rate of the fluid according to a load being an amount of heat required by the fluid utilization apparatus, the flow rate control device comprising: a load detection unit adapted to detect the load; and a flow rate control unit adapted to control the flow rate according to the load detected by the load detection unit, wherein the load detection unit includes: a post-temperature detection unit adapted to detect a post-temperature being a temperature of the fluid supplied to the fluid utilization apparatus (40, 50a-50c); a pre-temperature detection unit adapted to detect a pre-temperature being a temperature of the fluid returning from the fluid utilization apparatus (40, 50a-50c); and a temperature difference computation unit adapted to compute a difference between the post-temperature detected by the post-temperature detection unit and the pre-temperature detected by the temperature detection unit, characterized in that the flow rate control device further includes: a COP computation unit adapted to compute a COP (Coefficient Of Performance) based on the flow rate, the temperature difference, and power consumption of the heating/cooling apparatus, by COP = QΔT / W Q being the flow rate, W the power consumption of the fluid circuit system and ΔT the difference between the post-temperature and the pre-temperature. a heating/cooling apparatus for heating or cooling a fluid; a fluid utilization apparatus (40, 50a-50c) that is connected to the heating/cooling apparatus by a pipe and adapted to utilize the heated or cooled fluid and a flow rate control device adapted to control a flow rate of the fluid according to a load being an amount of heat required by the fluid utilization apparatus, the flow rate control device comprising: a load detection unit adapted to detect the load; and a flow rate control unit adapted to control the flow rate according to the load detected by the load detection unit, a load detection unit adapted to detect the load; and a flow rate control unit adapted to control the flow rate according to the load detected by the load detection unit, wherein the load detection unit includes: a post-temperature detection unit adapted to detect a post-temperature being a temperature of the fluid supplied to the fluid utilization apparatus (40, 50a-50c); a pre-temperature detection unit adapted to detect a pre-temperature being a temperature of the fluid returning from the fluid utilization apparatus (40, 50a-50c); and a temperature difference computation unit adapted to compute a difference between the post-temperature detected by the post-temperature detection unit and the pre-temperature detected by the temperature detection unit, a post-temperature detection unit adapted to detect a post-temperature being a temperature of the fluid supplied to the fluid utilization apparatus (40, 50a-50c); a pre-temperature detection unit adapted to detect a pre-temperature being a temperature of the fluid returning from the fluid utilization apparatus (40, 50a-50c); and a temperature difference computation unit adapted to compute a difference between the post-temperature detected by the post-temperature detection unit and the pre-temperature detected by the temperature detection unit, characterized in that the flow rate control device further includes: a COP computation unit adapted to compute a COP (Coefficient Of Performance) based on the flow rate, the temperature difference, and power consumption of the heating/cooling apparatus, by COP = QΔT / W Q being the flow rate, W the power consumption of the fluid circuit system and ΔT the difference between the post-temperature and the pre-temperature. a COP computation unit adapted to compute a COP (Coefficient Of Performance) based on the flow rate, the temperature difference, and power consumption of the heating/cooling apparatus, by COP = QΔT / W Q being the flow rate, W the power consumption of the fluid circuit system and ΔT the difference between the post-temperature and the pre-temperature.