A side airbag apparatus includes an inflator and an airbag. The airbag that is deployed and inflated in a space between a body side portion of a vehicle and an occupant seated on a front seat in a passenger compartment by the pressure of gas injected from the inflator. The apparatus also includes a pair of hook devices and tethers. The hook devices are located at the rear end in the airbag. Each tether extends in the fore-and-aft direction of the vehicle and has an end hooked to one of the hook devices and an end fixed to the front inner end of the airbag. The tethers suppress the expansion of the airbag in the fore-and-aft direction of the vehicle, thereby promoting the expansion of the airbag in the lateral direction of the vehicle. Further, the apparatus includes an occupant size detector for detecting the size of the occupant. When the size of the occupant detected by the occupant size detector is equal to or smaller than a reference size that is determined in advance, the hook devices release the tethers.