An air bag module mounted in front of a front passenger seat of an automotive vehicle. The air bag module includes a case having an opening. An air bag is accommodated within and connected with the case. The air bag is projectable out of the case through the opening of the case. A gas generator is provided for generating gas to be supplied into the air bag to inflate the air bag. A lid member is provided for covering the opening of the case. The case has a bottom wall section including a weakened portion which serves as a starting point of deformation of the case when an impact is applied to the case.