Arrangement of a vehicle seat and a vehicle side air bag device, said vehicle side air bag device (10; 70; 80) 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; and a side air bag (20; 72; 82) provided in a side portion of a seatback of the vehicle seat and 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 configured to restrain rear parts of the chest and the abdomen of the sitting occupant, the rear bag portion being configured such that a forward extending portion configured to restrain a shoulder of the sitting occupant is provided in an upper part thereof and the inflator is provided thereinside; the arrangement being characterized by further comprising: a longitudinal partition portion configured to partition the front bag portion from the rear bag portion, the longitudinal partition portion having a communicating opening configured to communicate an inside of the front bag portion with an inside of the rear bag portion; and an upper partition portion extending diagonally upward toward a front side of the seatback from an upper end of the longitudinal partition portion in an expanded and unfolded state, the upper partition portion being configured to partition the forward extending portion from the front bag portion and to go upward toward the front side relative to that front-rear direction of the seatback which is perpendicular to a height direction of the seatback in a seat side view. 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; and a side air bag (20; 72; 82) provided in a side portion of a seatback of the vehicle seat and 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 configured to restrain rear parts of the chest and the abdomen of the sitting occupant, the rear bag portion being configured such that a forward extending portion configured to restrain a shoulder of the sitting occupant is provided in an upper part thereof and the inflator is provided thereinside; a front bag portion_ configured to restrain front parts of a chest and an abdomen of the sitting occupant; a rear bag portion configured to restrain rear parts of the chest and the abdomen of the sitting occupant, the rear bag portion being configured such that a forward extending portion configured to restrain a shoulder of the sitting occupant is provided in an upper part thereof and the inflator is provided thereinside; the arrangement being characterized by further comprising: a longitudinal partition portion configured to partition the front bag portion from the rear bag portion, the longitudinal partition portion having a communicating opening configured to communicate an inside of the front bag portion with an inside of the rear bag portion; and an upper partition portion extending diagonally upward toward a front side of the seatback from an upper end of the longitudinal partition portion in an expanded and unfolded state, the upper partition portion being configured to partition the forward extending portion from the front bag portion and to go upward toward the front side relative to that front-rear direction of the seatback which is perpendicular to a height direction of the seatback in a seat side view. a longitudinal partition portion configured to partition the front bag portion from the rear bag portion, the longitudinal partition portion having a communicating opening configured to communicate an inside of the front bag portion with an inside of the rear bag portion; and an upper partition portion extending diagonally upward toward a front side of the seatback from an upper end of the longitudinal partition portion in an expanded and unfolded state, the upper partition portion being configured to partition the forward extending portion from the front bag portion and to go upward toward the front side relative to that front-rear direction of the seatback which is perpendicular to a height direction of the seatback in a seat side view.