To provide a flow rate calculation circuit for an open type channel that unnecessitates adjustment by a variable resistor, and that can stably obtain the flow rate of the open type channel.

A switch 4 outputs either a flow rate signal V4 calculated based on a water level analogue signal V1 outputted from a water level measurement device 2, a reference voltage V3 and a water level, or a zero electric potential V2 where none of them is present as an analogue signal. An A/D(analogue/ digital) converter 6 digitized the analogue signal for outputting as a first digital data, and a conversion device 7 performs a specified calculation using the first digital data for outputting a second digital data, then a D/A(digital/analogue) converter 8 converts the second digital data to analogue value for outputting a flow rate signal V4.