A side airbag unit for a vehicle, constructed to protect a side of a passenger in the event of a side collision or when the vehicle overturns, the side airbag unit may include an airbag module having a stud which is inserted into a predetermined portion of a seat frame, and an airbag cushion which remains folded in the airbag module and expands when the airbag unit is deployed, and a strap allowing the airbag cushion to be deployed to a predetermined length at a predetermined position when the airbag cushion expands, wherein a first end of the strap is coupled to a predetermined portion of the airbag cushion and a second end thereof is coupled to the stud such that the second end of the strap is being locked to the stud when the stud is assembled with the seat frame.