A side airbag apparatus for a vehicle is provided. The apparatus has an airbag and a gas generator. The gas generator supplies gas to the airbag such that the airbag is deployed and inflated at a side of an occupant. The airbag has a loop seam, which is formed by sewing parts of the airbag that face each other in a lateral direction of the vehicle such that no clearance is created at the loop seam. A slit formation portion is provided in part of the airbag that is encircled by the loop seam. When the airbag is deployed and inflated, the slit formation portion opens as a slit.