Elastic members are retained in respective recesses of open boxes. Heating (cooling) temperature sensing elements are held on the elastic member. A pipe which has already been laid is sandwiched between the open boxes such that the heating (cooling) temperature sensing elements join to desired positions, respectively, of the existing pipe. The pipe is subjected to a temperature change by the heating (cooling) temperature sensing elements, and hence the fluid therein is subjected to a temperature change, causing a temperature gradient to occur between the upper and lower streams of the fluid flow. The temperature gradient is detected by the heating (cooling) temperature sensing elements, and a flow rate of the fluid flowing through the pipe is obtained on the basis of the outputs of the sensing elements.