An inflation portion of an airbag, which is inflated by inflation gas, is partitioned into an upstream inflation portion and a downstream inflation portion via a partition member. When being tensioned in a planar manner, the partition member has a length in a longitudinal direction that is longer than a length in a transverse direction perpendicular to the longitudinal direction. A pressure regulating valve that is closed at an early stage of a supply period of the inflation gas to the inflation portion and is opened from the middle of the supply period includes a slit-like inner opening that is provided on the partition member and extends in the transverse direction and a pair of valve body portions that are provided around the inner opening and are selectively positioned close to and separated from each other.