A side bolster adjusting apparatus of a seat of a vehicle for guiding a developing direction of a side airbag may include a side airbag developing direction guide plates integrally fixed to a seatback frame to guide a developing direction of the side airbag, side bolster adjusting plates hinged to rear ends of the side airbag developing direction guide plates, an airbag disposed between the side airbag developing direction guide plates and the side bolster adjusting plates to expand and shrink and to adjust positions of the side bolster adjusting plates, and a pneumatic control unit controlling expansion and shrinkage of the airbag.