PURPOSE: To unitize an air bag unit inclusive of an acceleration sensor as well as to enhance the extent of reliability.

CONSTITUTION: An air bag 5, an inflater 6 and an acceleration sensor are all intensively set up at one spot, constituting an air bag unit. Two acceleration sensors 7, 8 are installed in this unit, while taking account of a fact that these sensors are placed under the same circumstances by means of unitization, these sensors are set down to those different in kind with one another, whereby these sensors are made so as not to get out of order at the same time. When the air bag unit is attached to a steering wheel via a retainer 4 or the like, these sensors 7, 8 are set up so as to make the traveling direction of each needle, being provided in them, parallel a steering axis.