PURPOSE: To measure quickly an exact flow rate without using a large structure, by calculating an open channel flowing water sectional area from an open channel water-level signal, and calculating an actual mean flow velocity from an output signal of an open channel water-level manual setting device and an open channel signal.

CONSTITUTION: An output signal of a water-level transmitter 2 in an open channel 1 is converted to an open channel signal 4 by a water-level transducer 3, is inputted to a function generator 5, and a flowing water sectional area signal 6 of the open channel is calculated and is outputted to a multiplier 7. An output signal 10 of a manual setting device 9 which sets an open channel water-level of a full scale is inputted to a divider 8, the signal 4 inputted separately is divided by the signal 10, and a divided signal 11 is outputted to a function generator 12. The generator 12 inputs the signal 11 and outputs a flow velocity correcting signal 13 to a divider 14. An output signal of a flow velocity transmitter 15 existing in a mean flow velocity position of the full scale open channel water-level is converted to a flow velocity signal 17 by a flow velocity transducer 16, the signal 17 is inputted to the divider 14, is divided by the signal 13, and an actual mean flow velocity signal 18 is outputted to the multiplier 7. The multiplier 7 executes multiplication of the signals 6, 18 and an open channel flow rate signal 19 is outputted.