A seat occupant determining apparatus includes a first load detecting sensor at a right of a seat, a second load detecting sensor at a left of the seat, a third load detecting sensor at a front or a rear of the first and second load detecting sensors, in order to detect a part of the load, respectively, a right-left sum value calculating portion calculating a right-left sum value, a threshold changing portion for changing an occupant determining threshold on the basis of the load value of the third load detecting sensor, and an occupant determining portion determining an occupant of the seat to be an adult when the right-left sum value is equal to or more than the occupant determining threshold and determining an occupant of the seat to be a child in a child seat when the right-left sum value is less than the occupant determining threshold.