PURPOSE: To eliminate the generation of fluctuation of a flow rate at the time of a large rate, and measure the flow rate stably by fitting a discharge pipe to be fitted to a flow meter so that discharge port thereof is positioned higher than the flow meter and that the fluid is discharged to the direction lower than the level.

CONSTITUTION: A discharge pipe 13, which is bent twice at 90 degrees each time so that both end surfaces thereof are parallel to each other, is fitted to a flow meter 12 provided in the lower part of a weir board 1 so that the axis of a discharge port thereof is directed horizontally and that the discharge port is positioned higher than the flow meter 2. Under the condition with a small flow rate, Δh is stabilized to enable the stable measurement of the flow rate. Under the condition with a large flow rate, spounting for discharging the fluid from the discharge port to the direction lower than the level is not generated, and since the water head H of the discharge flow at this time is decided in response to the height of the discharge port, instability of the rate due to the condition of the discharge flow is not generated. Consequently, the flow rate can be measured stably even at the time of the large flow rate.