A seat apparatus for a vehicle having a side support portion that is controlled depending on a road condition, includes a distance calculating means for calculating a distance of a straight portion between a first curve, on which a vehicle is supposed to be driven, and a second curve, on which the vehicle is supposed to be driven after the first curve, based on an electronic map data, a first controlling level calculating means for calculating a first controlling level at which the side support portion is controlled when the vehicle is driven on the first curve, a second controlling level calculating means for calculating a second controlling level at which the side support portion is controlled when the vehicle is driven on the straight portion, and a driving means for operating the side support portion based on the first controlling level and the second controlling level.