A passenger detecting device includes a plurality of weight sensors, a passenger judging and data updating means and an update forbidding 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 passenger judging and data updating means is capable of judging a passenger on the seat based on the loads detected by the weight sensors and updating information on the passenger. The update forbidding means prevents the passenger judging and data updating means from judging the passenger or prevents the information to be updated when a sum-weight fluctuation amount is not less than a first predetermined threshold value, where the sum-weight fluctuation amount is obtained by summing up weight function amounts which are absolute values of fluctuations of the loads detected by the weight sensors.