A weight sensing device providing an accurate measurement of only a passenger's vertical load without an interference of moment and lateral load caused upon sudden braking and a variation in traveling conditions of a vehicle. The weight sensing device includes a sensor body having a center hole for fastening of the sensor body with a seat frame, a diaphragm provided around the center hole, a pair of fixing holes provided at opposite sides of the center hole and arranged coaxially with the center hole for fastening of the sensor body with a slide rail, supporting planes containing the fixing holes and located at opposite sides of the diaphragm, and a mounting recess defined between the diaphragm and the supporting planes, a semiconductor strain gauge and a printed circuit board attached to the diaphragm of the sensor body, and a connector electrically connected with the printed circuit board, to transmit a value output from the semiconductor strain gauge to an airbag controller.