The airbag of the present invention, when completely inflated, is formed into a generally square conical shape whose top is in a front end of the airbag. A protection portion to be located in rear side of the airbag upon airbag inflation includes shoulder restraining portions disposed generally side by side in left-right orientation in an area from upper part to rear part of the completely inflated airbag. Each of the shoulder restraining portions continuously projects in upper part to rear part of the airbag.