PURPOSE: To absorb an impact at the time of expansion of an air bag against a rider in this improper position by adjusting a spread position of the air bag to the more upper position than that at usual time at a time when such a state that a child or the like is not seated and uprighted as adjoining an assistant driver's seat is detected.

CONSTITUTION: In an air bag unit 20, its casing 22 is supported on a bracket 16 locked to a steering support member 15 free of rotation, while it is constituted to be rotatable with a motor 26. In addition, a control circuit 28 is connected to this motor 26, while a proximity sensor 29 detecting a following standing child state and a seat switch 30 are connected each to this control circuit 28. When the standing child state, namely, such a state that a child is not seated and uprighted as adjoining an assistant driver's seat is detected, the casing 22 is rotated at the more upper position than that at usual time. With this constitution, any possible impact against the child at time of expansion of the air bag is absorbed in this way.