An apparatus for determining a seat occupancy includes a load detecting portion attached to a seat portion of a seat for a vehicle and detecting a seat occupancy load, a seatbelt buckling detecting portion detecting an engagement of a buckle of a seatbelt, and a seat occupancy determining portion determining the seat occupancy on the basis of detection signals from the seatbelt buckling detecting portion and the load detecting portion. The seat occupancy determining portion determines a mounted state of a child seat, in which the child seat is placed on the seat and is fixed thereto by means of the seatbelt, in a case where the seat occupancy load changes so as to be increased at a predetermined speed or faster and subsequently so as to be decreased at a predetermined speed or faster, within a time period proximate to a time point when the buckle is engaged.