A side extension device for a seat of a vehicle, may include a seat cushion frame for forming a framework, a cushion pad provided with a bolster formed on each of opposite sides thereof, and a seat cover for covering a surface of the cushion pad, wherein a plurality of slits is formed in the bolster of the cushion pad, and divided bolster parts are formed between the slits, wherein the divided bolster parts are successively inclined outward by contact pressure of side parts of hips of an occupant, so that the bolster spreads outward.