PURPOSE: To omit a calculational work to be performed based on an arithmetic expression and the generation control work of a polygonal data to be supplied to a converter by converting the water level value into the flow rate value based on a flow rate arithmetic expression that is selected by the shape of a weir.

CONSTITUTION: An A-D conversion part 2 converts the water level analog signal showing the water level value of the weir into a water level digital signal. A flow rate arithmetic expression selection part 8 is connected to the part 2 and selects a flow rate arithmetic expression based on the shape of the weir outputted from the part 2. A control arithmetic part 1 is connected to the parts 2 and 8 and converts the water level value into the flow rate value based on the water level digital signal outputted from the part 2 and the flow rate arithmetic expression selected by the part 8. A D-A conversion part 4 is connected to the part 1 and converts the flow rate digital signal outputted from the part 1 into a flow rate analog signal. Then a display part 3 is connected to the part 1 and displays the flow rate of the weir based on the flow rate analog signal outputted from the part 4.