Abstract: An apparatus for controlling a flow rate of fluid passing through a duct (10) includes a wedge (12) to produce a resistance against a flow of the fluid; pressure sensors (20-1, 20-2), to detect an inflow pressure and an outflow pressure at the upstream and the downstream of the duct (10), respectively, a flow rate controller (22) for measuring a flow rate corresponding to a differential pressure between the inflow and the outflow pressures, comparing the measured flow rate with a predetermined flow rate to regulate an opening angle at an outflow side of the duct (10) using a calculated opening angle, wherein the calculated opening angle being corresponded to a deviation of flow rate between the measured flow rate and the predetermined flow rate; and an error detector (28) for detecting an improper increase of the differential pressure to correctly adjust the opening angle regulated by the flow rate controller.