Head protecting airbag device

A passenger's seat airbag device of the invention is mounted on a vehicle such as a single-cab vehicle (V1) in which a rear wall portion (12) is so arranged on the rear side of a side window (SW) as to intersect the side window (SW) generally perpendicularly. The airbag device includes an airbag (20) which is so folded and housed on the upper edge side (RR) of the side window (SW) on the interior side as to cover the side window (SW) at its extending and expanding time. The airbag (20) is mounted on the vehicle body on its two longitudinal ends. The rear side leading end is mounted on a position of the rear wall portion away from the vehicle side face. In the vicinity of the corner portion of the vehicle, there is arranged guide means (42) which can guide the downward protrusion of the airbag (20) when the airbag (20) is extending and expanding and can regulate the movement of the airbag (20) to the interior side when the airbag (20) is extending and expanding.