PURPOSE: To facilitate the maintenance and repair while reducing the cost by calculating the pressure at a branch point based on an accurate flow rate of main pipe, calculating the pressure drop of branch pipe based on the calculation results and a flow rate signal from each flow rate detector for branch pipe, and then calculating the flow rate of gas flowing through the branch pipe accurately, thereby eliminating pressure detector for each branch pipe.

CONSTITUTION: A pressure correction operating means 7 inputs a pressure signal SP along with a differential pressure signal SFO from a main pipe flow rate detector and outputs a main pipe flow rate FLO, and at the same time, determines the pressure at a branch 5 based on the SP and FLO. Pressure at the branch point 5 is operated again along with differential pressure signals SF1,..., SF3 from flow rate detectors 2-1,..., 2-3 and an operation result (smi) is delivered, as a pressure signal for correction, to a calculator 8. In the calculator 8, differential signals from the detectors 2-1,..., 2-3 are fed to an extraction unit 9 which outputs corrected branch flow rates FL1,..., FL-3. Consequently, the pressure can be corrected even if pressure detector for pressure correction is eliminated from each branch pipe.