A passenger protection device includes an airbag case to be provided in an arm rest of a vehicle seat, and an airbag; wherein the airbag case has a passenger-side wall that is a cover that can rotate upward around its upper end; the device comprises support members that lie flat on a passenger-side surface of the cover and can assume an upright orientation; and if the vehicle collides at one side, the airbag is inflated and deployed outside the airbag case through the opening made in a passenger-side of the arm rest and pushes the cover, thereby rotating the cover upward, the support members may stand on the airbag case, supporting the cover rotated upward from back in an upright orientation.