A vehicle side air bag device comprising: an inflator provided in a vehicle seat and configured to be activated to generate gas when a side collision of a vehicle is detected or predicted; a side air bag provided in a side portion on an outer side, in a vehicle width direction, of a seatback of the vehicle seat, the side air bag being configured to be expanded and unfolded between a sitting occupant and a vehicle-body side portion when the gas generated from the inflator is supplied thereinside, the side air bag including a front bag portion configured to restrain front parts of a chest and an abdomen of the sitting occupant, a rear bag portion including the inflator thereinside and configured to restrain rear parts of the chest and the abdominal, and a forward extending portion extended toward a vehicle front side from an upper end of the rear bag portion so as to be placed on a vehicle upper side of the front bag portion, the forward extending portion being configured to restrain a shoulder of the sitting occupant; a tether portion placed in a boundary portion between the front bag portion and the rear bag portion in the side air bag so as to partition the front bag portion from the rear bag portion, the tether portion having a gas feed opening via which the front bag portion and the rear bag portion communicate with each other; and a seam portion or a narrow-width tether portion narrower than the tether portion, the seam portion or the narrow-width tether portion being set in a boundary portion between the forward extending portion and the front bag portion in the side air bag so as to partition the forward extending portion from the front bag portion, wherein the seam portion or the narrow-width tether portion extends from an upper end of the tether portion toward the vehicle front side and a vehicle diagonally upper side, and is inclined forward in a side view, the side air bag is configured to include: a main panel made of one or two panels and forming the rear bag portion, the forward extending portion, and the tether portion; and paired right and left subpanels sewed to the main panel with tether margins to form the tether portion, the paired right and left subpanels forming the front bag portion. an inflator provided in a vehicle seat and configured to be activated to generate gas when a side collision of a vehicle is detected or predicted; a side air bag provided in a side portion on an outer side, in a vehicle width direction, of a seatback of the vehicle seat, the side air bag being configured to be expanded and unfolded between a sitting occupant and a vehicle-body side portion when the gas generated from the inflator is supplied thereinside, the side air bag including a front bag portion configured to restrain front parts of a chest and an abdomen of the sitting occupant, a rear bag portion including the inflator thereinside and configured to restrain rear parts of the chest and the abdominal, and a forward extending portion extended toward a vehicle front side from an upper end of the rear bag portion so as to be placed on a vehicle upper side of the front bag portion, the forward extending portion being configured to restrain a shoulder of the sitting occupant; a tether portion placed in a boundary portion between the front bag portion and the rear bag portion in the side air bag so as to partition the front bag portion from the rear bag portion, the tether portion having a gas feed opening via which the front bag portion and the rear bag portion communicate with each other; and a seam portion or a narrow-width tether portion narrower than the tether portion, the seam portion or the narrow-width tether portion being set in a boundary portion between the forward extending portion and the front bag portion in the side air bag so as to partition the forward extending portion from the front bag portion, wherein the seam portion or the narrow-width tether portion extends from an upper end of the tether portion toward the vehicle front side and a vehicle diagonally upper side, and is inclined forward in a side view, the side air bag is configured to include: a main panel made of one or two panels and forming the rear bag portion, the forward extending portion, and the tether portion; and paired right and left subpanels sewed to the main panel with tether margins to form the tether portion, the paired right and left subpanels forming the front bag portion. a main panel made of one or two panels and forming the rear bag portion, the forward extending portion, and the tether portion; and paired right and left subpanels sewed to the main panel with tether margins to form the tether portion, the paired right and left subpanels forming the front bag portion.