A passenger detecting device includes a plurality of weight sensors, a weight-fluctuation monitoring means and a passenger judging and data updating means. The weight sensors are arranged at different positions of a seat for respectively detecting weights acting on the seat of a motor vehicle. The weight-fluctuation monitoring means monitors weight-fluctuation amounts of the plurality of the weight sensors and sum-weight fluctuation amount. The passenger judging and data updating means judges a passenger on the seat to be a large physical-size person or a small physical-size person to update information on the passenger on the seat when the sum-weight fluctuation amount is not less than each of the weight fluctuation amounts, and prevents the information to be updated when at least one of the weight fluctuation amounts is larger than the sum-weight fluctuation amount.