An occupant determining device for a vehicle seat includes a tension detecting means for detecting a tension of a seat belt, a load detecting means for detecting a load applied to the vehicle seat, a determining means for determining an occupant on the vehicle seat and a tightened state of the seat belt based on either one or both of a detected result of the tension detecting means and the load detecting means, and a display means for displaying a determination result of the tightened state of the seat belt.