An air bag device for a vehicle provided with at least two rows of seats including a front seat and a rear seat arranged in a front-rear direction, and configured such that relative positions of the front seat and the rear seat in the front-rear direction are changeable, the air bag device being mountable on a rear surface of a seatback of at least the front seat, comprising: an air bag configured to be deployed rearwardly into a shape having a cavity portion therein in a vehicle side view by gas to be supplied from an inflator, the air bag including shape holding means configured to hold the air bag itself in a shape having the cavity portion therein against a gas pressure of an inner space of the air bag, when being deployed, characterized in that the air bag includes an outer peripheral portion which constitutes an outer periphery of the air bag, and an inner peripheral portion which defines the inner space in cooperation with the outer peripheral portion, and constitutes an inner periphery of the air bag, and the shape holding means has two partition walls (14L, 14R) for separating the inner space into three divided spaces (11A, 12A, 13A) arranged in a vehicle width direction by connecting the outer peripheral portion and the inner peripheral portion within the inner space of the air bag at respective predetermined positions in the vehicle width direction, each of the partition walls is formed with a communication hole communicating the middle divided space with the right and left divided spaces (12A, 13A) of the three divided spaces, the airbag is provided with a gas inlet port for supplying the gas from an inflator into the middle divided space and a vent hole for discharging gas from the right and left divided spaces (12A, 13A) outwardly in vehicle width directions, respectively. an air bag configured to be deployed rearwardly into a shape having a cavity portion therein in a vehicle side view by gas to be supplied from an inflator, the air bag including shape holding means configured to hold the air bag itself in a shape having the cavity portion therein against a gas pressure of an inner space of the air bag, when being deployed, characterized in that the air bag includes an outer peripheral portion which constitutes an outer periphery of the air bag, and an inner peripheral portion which defines the inner space in cooperation with the outer peripheral portion, and constitutes an inner periphery of the air bag, and the shape holding means has two partition walls (14L, 14R) for separating the inner space into three divided spaces (11A, 12A, 13A) arranged in a vehicle width direction by connecting the outer peripheral portion and the inner peripheral portion within the inner space of the air bag at respective predetermined positions in the vehicle width direction, each of the partition walls is formed with a communication hole communicating the middle divided space with the right and left divided spaces (12A, 13A) of the three divided spaces, the airbag is provided with a gas inlet port for supplying the gas from an inflator into the middle divided space and a vent hole for discharging gas from the right and left divided spaces (12A, 13A) outwardly in vehicle width directions, respectively.