A seat apparatus for a vehicle includes a load detection device placed at a front and a rear of one of a right-left pair of the attachment members so as to be spaced apart from each other, the load detection device measuring a load applied to a seat to distinguish an existence of a passenger and a type of a passenger, a detection influence determination device determining an existence of an influence on a passenger determination performance of the load detection device in a case where a collision load value detected by the load detection device in the case of a collision of the vehicle corresponds to a detected value between a preset first collision load value and a preset second collision load value, and a collision influence alert device outputting an alert in a case where the detection influence determination device determines that the passenger determination performance is influenced.