A seat-mounted airbag device includes a guidance member and an airbag body which includes a front-rear chamber and a distal-end chamber and is accommodated in a side portion of a headrest of a window-side vehicle seat in a state where the airbag body includes an outward winding portion. At the time of a front end collision, the front-rear chamber deploys from the side portion through a gap between the head of an occupant sitting on the window-side vehicle seat and the head of an occupant sitting on a central-side vehicle seat and is disposed in the gap. The distal-end chamber deploys inwardly from a front end part of the front-rear chamber. The guidance member projects from the side portion prior to the deployment of the airbag body so as to be disposed between the airbag body and the head of the occupant sitting on the central-side vehicle seat.