A fastening strap unit includes a frame with a mandrel and a strap connected thereto. The strap is wrapped to the mandrel, and the first end of the strap is fixed to the mandrel and a hook is connected to the second end of the strap. A shaft extends from the frame and a motor drives the shaft which drives the mandrel. A first sensing chip is connected to the second end of the strap and a second sensing chip is connected to the first end of the strap. A sensor is connected to the frame so as to detect the first sensing chip or the second sensing chip when the strap is retracted or released to its final position, and the motor is stopped.