An occupant weight detecting device includes a load detector provided at a seat body and detecting load applied to the seat body and an occupant detector for detecting an occupant sitting on the seat body based on the load detected by the load detector. The occupant detector includes a load calculator for calculating the load based on a detected value detected by the load detector, a load area determination device for determining a load area by comparing the load with a predetermined threshold, and an occupant determination device for determining a type of occupant sitting on the seat body based on the load area.