A seat load determining apparatus, which includes a load sensor provided at least one rear support portion which support a seat and detecting a load applied to the rear support portion, a load detection portion detecting a part of a weight of a load applying body provided at the seat and outputting a detected load value, a vehicle start detection sensor detecting that a vehicle starts moving, and a determining portion determining that the load applying body corresponds to an adult when the load detection portion detects the detected load value to be greater than a first determination value within a predetermined time, the determining portion determining that the load applying body corresponds to a child when the load detection portion detects the detected load value that is smaller than the first determination value and greater than a second determination value which is smaller than the first determination value.