An apparatus for determining seat occupancy includes a seat for a vehicle, which has a seating portion, right and left load detecting portions arranged between a vehicle floor and either one of a front portion and a rear portion of the seating portion, the right load detecting portion detecting a load applied to a right side of the seating portion, the left load detecting portion detecting a load applied to a left side of the seating portion, and a seat occupancy determining portion calculating a seating load of the seat on the basis of the detected loads and determining the seat occupancy of the seat by comparing the seating load with a predetermined threshold value, wherein the seat occupancy determining portion varies the predetermined threshold value depending on of a difference value between the loads applied to the right and left sides, respectively, of the seating portion.