An airbag housed in the housing and inflatable with an inflation gas for deployment rearward in order to protect a passenger in a front passenger seat. The airbag includes a main bag section that includes at its rear surface a front-collision arresting plane for catching the passenger upon a frontal collision, and an auxiliary bag section that includes an oblique-collision arresting section protruding rearward relative to the main bag section. The oblique-collision arresting section includes on its lateral an oblique-collision arresting plane for catching a head of the passenger upon an oblique collision. The oblique-collision arresting section is a high-pressure section which has a higher internal pressure than the main bag section when inflated. The airbag further includes in a front region of the auxiliary bag section a low-pressure section that is in gas communication with the main bag section and partitioned from the high-pressure section by a partition wall.