To provide a fluid flow rate measuring instrument which can stably measure differential pressure and obtain an accurate flow rate measured value of fluid without being affected by dynamic pressure or a vortex even when a flow straightening distance can not be secured on a venturi upstream side because of restrictions of the layout of a fluid flow passage.

A throat extension part where primary air 13 having passed through a throat part 42 is circulated branching into two flows having the same speed as it is and a core type flow passage partitioning member 62 having a 1st gradual expansion part 60 and a 2nd gradual expansion part 61 formed extending gradually increasing flow passage sectional area continuously from the throat extension part 59 are arranged in the skirt part 42b of a venturi 42; and the flow rate of the primary air 13 is found according to the differential pressure ΔP between the pressure P2' of the throat extension part 59 and the pressure P3 at the exit part of the skirt part 42b of the venturi.