A vehicle side airbag device comprising a side airbag provided at a seat and having at least a shoulder protection part and a chest protection part, the side airbag being inflated and deployed by inflator gas forward in a front-back direction of a vehicle body between the seat and a vehicle body member on a side of the seat, wherein when the side airbag is provided at the seat and spread out flat in a non-inflated state, a contour of the side airbag on a front side in the vehicle body front-back direction has, in a view in a widthwise direction of the vehicle body, an inflection position with a curvature change midway between the shoulder protection part and the chest protection part, an upper edge line oriented upward from the inflection position, and a lower edge line oriented downward from the inflection position, the inflection position is positioned ahead of the shoulder protection part and above a center line, the center line extending in a lengthwise direction of an upper arm of an occupant represented by a test dummy according to World SID 50 th, and dividing the upper arm into upper and lower portions (Au) in a view in the vehicle body widthwise direction, and the lower edge line crosses the center line from an upper side to a lower side and is positioned under the center line in a lower end position of the upper arm of the World SID 50 th occupant characterized in that the side airbag includes an inner bag therein, the inner bag is provided on a back side in the vehicle body front-back direction so that a main part thereof is positioned inside the seat in an inflated and deployed state, and the inner bag is provided with an inflator therein, and the inner bag is provided with a vent valve opened in response to increase in the internal pressure to bring inside of the inner bag and the inside of the side airbag into communication, and inflator gas introduced into the inner bag is introduced into the side airbag with time delay. when the side airbag is provided at the seat and spread out flat in a non-inflated state, a contour of the side airbag on a front side in the vehicle body front-back direction has, in a view in a widthwise direction of the vehicle body, an inflection position with a curvature change midway between the shoulder protection part and the chest protection part, an upper edge line oriented upward from the inflection position, and a lower edge line oriented downward from the inflection position, the inflection position is positioned ahead of the shoulder protection part and above a center line, the center line extending in a lengthwise direction of an upper arm of an occupant represented by a test dummy according to World SID 50 th, and dividing the upper arm into upper and lower portions (Au) in a view in the vehicle body widthwise direction, and the lower edge line crosses the center line from an upper side to a lower side and is positioned under the center line in a lower end position of the upper arm of the World SID 50 th occupant characterized in that the side airbag includes an inner bag therein, the inner bag is provided on a back side in the vehicle body front-back direction so that a main part thereof is positioned inside the seat in an inflated and deployed state, and the inner bag is provided with an inflator therein, and the inner bag is provided with a vent valve opened in response to increase in the internal pressure to bring inside of the inner bag and the inside of the side airbag into communication, and inflator gas introduced into the inner bag is introduced into the side airbag with time delay. the inner bag is provided on a back side in the vehicle body front-back direction so that a main part thereof is positioned inside the seat in an inflated and deployed state, and the inner bag is provided with an inflator therein, and the inner bag is provided with a vent valve opened in response to increase in the internal pressure to bring inside of the inner bag and the inside of the side airbag into communication, and inflator gas introduced into the inner bag is introduced into the side airbag with time delay.