PURPOSE: To sufficiently restrain a driver after an air bag is developed, by installing a means for preventing the expulsion by pressing the leg part of the driver from outside to the inside after the development of the air bag is completed and a means for restraining the vicinity of the center of gravity position of the driver.

CONSTITUTION: When collision occurs, a collision sensor senses collision, and sends signals into an air bag 5 and an operating switch 14. Then, the air bag 5 is developed, and performs initial restraint for a driver. While, the operating switch 14 pulls out an arm lock pin 13 and separates an arm 3a from a frame 6. Then, the arm 3a starts turn by the strong springy forces of springs 9 and 10. When the internal pressure of the air bag 5 lowers, and the behavior of the driver becomes unstable, a pad lock pin 18 is pulled out from the second shaft 15 and a pad side bearing 17 by being pulled out by a wire 19 whose one edge is fixed with the frame 16. Accordingly, the pad 4a pulls the left and right thigh parts of the driver inside, pressing the thigh parts to the upper surface of a seat by the strong springy force of a spring 16, and the correction to the stable attitude is prevented.