A fuel flow rate measuring device comprising a control rod axially movable in proportion to the rate of flow of air flowing through a suction pipe, a ball disposed in a taper bore and adapted to be moved in the taper bore by the control rod, the taper bore and the ball cooperating with each other to define a clearance which is variable in proportion to the rate of flow of the air, and a pressure regulator provided between a fluid inlet and outlet for keeping constant the fluid pressure difference across the clearance defined by the taper bore and ball.