In an airbag, a distal side is inflated away from a proximal side with an inflator disposed on the proximal side. The airbag includes a left airbag section, a right airbag section, and a belt strap disposed in one of the left and right airbag sections located closer to a vehicle body side for connecting a portion of the airbag section on the vehicle body side and an opposite portion. A height H1 in a vertical cross section from a lowest level to a joint portion between the belt strap and the portion is within a range from 0.4 H to 0.8 H, wherein H represents an entire height. A height H2 from the joint portion between the belt strap and the opposite portion is within a range from 0.3 H to 0.6 H, and is smaller than H1.