PURPOSE: To measure the flow rate at a place where measurement is hard to perform, by providing an orifice, whose top side forming an isosceles triangular prism is directed toward the upstream side, and which is arranged in the longitudinal direction of a flow path so that the axial center crosses the flow at a right angle, and providing means which detect the pressures at the upstream side and the downstream side.

CONSTITUTION: In an orifice 10 forming an isosceles triangular prism, sides 10a and 10b are equal, and a top side 10c is directed toward the upstream side. Pressure detecting holes 11 are guided in the orifice 10 and opened at a bottom surface 10e. Pressure introducing pipes 12 are connected to the holes 11, respectively. The top side 10c of the orifice 10 is directed toward the upstream side of flow paths 7 which are partitioned by separating walls 6 of a duct 1. The axis of the orifice is directed in the longitudinal direction with regard to the cross section of the flow path. Differential pressure gages are attached to pipes 12 and 13. The relationship between the differential pressure of the orifice 10 and the flow rate (flow speed) is calibrated in advance. Thus the low rate in the flow path 7 is obtained. In the case, the flow rate is accurately measured by only inserting the orifices through opening parts provided in the wall of the duct 1.