In a vehicle side air bag device, an air bag is fixed to a side frame portion of a seatback frame via an inflator at a fixed part set in a rear end of a vertically intermediate part. An upper insertion portion into which an upper end of a reaction plate is inserted is formed on an upper-part side in a rear end of the air bag, and a lower insertion portion into which a lower end of the reaction plate is inserted is formed on a lower-part side in the rear end of the air bag. The reaction plate is modularized with the air bag, the inflator, and so on, and is fixed to the side frame portion via the inflator.