A central member of a sensor unit is inserted through a sensor unit mounting hole of a cylindrical body having a main passage and the sensor unit is assembled on the cylindrical body. The sensor unit comprises the central member which is provided at the center of the main passage and has internally a bypass passage, a flow sensor provided in the bypass passage, a rib which supports the central member, a mounting section which secures the rib to the cylindrical body, an electronic circuit assembled on the rib, and a connector section retaining terminals being connected electrically to the electronic circuit section. Positioning, mounting and securing of the flow sensor can be made simultaneously by a simple process to insert the sensor sensor unit through the hole. Turbulence, eccentricity and others in the upstream air are reduced so that the flow rate of a highly rectified fluid can be measured.