An airbag comprises a right half airbag to be inflated on a front right side of an occupant, a left half airbag to be inflated on a front left side of the occupant, and a base-end chamber that communicates with base-end sides of the right half airbag and the left half airbag. A vertically extended concave portion exists on a surface of the inflated airbag that faces the occupant. The concave portion is deepest in the intermediate part and shallow in the upper and lower parts.