A flap position sensing device of an active air flap system, and a method of manufacturing the same are described. The flap position sensing device includes a rotating body, a sensor casing, a first hall sensor and a second hall sensor, and a first permanent magnet and a second permanent magnet. The rotating body is formed of a resin material, and rotates the flap by receiving torque while the rotating body is coaxially fixed to a rotation shaft of the flap. The sensor casing is formed of a resin material, and is fixed to the housing to rotatably support the rotating body. The first hall sensor and the second hall sensor are integrally formed with the sensor casing by injection molding, and are disposed corresponding to an open position and a closed position, respectively, of the flap.