A vehicle side air bag apparatus includes: a bag that is housed within a side portion outside in a seat back in a vehicle width direction and inflates and expands to a vehicle front side of the side portion by supplying gas; an up-down partition that partitions the bag into lower and upper bag portions; a right-left partition which partitions the lower bag portion into left and right chambers, to which a right-left communication port is provided and which inhibits inflation of the lower bag portion in an upper and lower direction by being extended in the inflation and expansion state; an inflator provided in the upper bag portion; and a flow regulating portion that distributes gas into one of the left and right chambers and the upper bag portion and limits gas flow from the one chambers to the upper bag portion by a check valve portion.