An airbag device for a vehicle includes a gas supply portion, an airbag body, and a low-pressure portion. When a vehicle collision occurs, the gas supply portion injects gas into an airbag body. The airbag body has a single-chamber structure. When the airbag body receives the gas from the gas supply portion, the airbag body is inflated and deployed toward an occupant to restrain the shoulders of the occupant. The low-pressure portion is provided in the same area of the airbag body, which faces the occupant when the airbag body is inflated and deployed, to protect the occupant's head. When the airbag body is inflated and deployed, the low-pressure portion is inflated such that the pressure in the low-pressure portion is lower than the pressure in the airbag body.