PURPOSE:To measure the flow rates accurately, by arranging the first and second pipes having different ratios between a differential pressure component of a granular material and a differential pressure component of a conveying gas, at the intermediate part of a pipe, in which the granular material is conveyed by the conveying gas, in series. CONSTITUTION:At the intermediate part of a pipe 2, in which a granular material is conveyed by a coneying gas, pipes 3 and 4 having different ratios between a differential pressure component of the granular material and a differential pressure component of the conveying gas are arranged in series. The signals of differential pressures DELTAP1 and DELTAP2 that are measured by differential pressure gages 5 and 8, pressures P1 and P2 and a temperature T1 are inputted to an operator 10. Then, the flow rate of the granular material, which is conveyed in the pipe 2, and the flow rate of the conveying gas are operated. At this time, operations are performed by expressions DELTAP1=a11Ga+a12Gs+a13P1+a14T1+a15, and DELTAP2=a21Ga+a22Gs+a23P2+a24T1+a25. Thus the flow rates can be accurately measured.
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