PURPOSE: To enhance the safety of an occupant at the time of collision, which produces a lateral load exceeding the prescribed load, by preventing a head part of the occupant from moving.

CONSTITUTION: A portion of a seat belt 10, which is diagonally placed from a shoulder part to a waist part of the occupant, is formed into the form of hollow, airtight bag, and the hollow part is connected to a gas pressure generator 24. This gas pressure generator 24 is connected to a collision detecting means 26. When the vehicle body is in a collision, the collision detecting means 26 operates so that the gas pressure is supplied from the gas pressure generator 24 to the hollow part of the seat belt 40. As a result, a part of the seat belt 10, which is arranged in the shoulder part of an occupant 22 is instantly inflated. Thus, the restricting force of the seat belt 10 to the occupant 22 is increased, thereby preventing the occupant 22 from sidelong falling. At the same time, the inflated part of the seat belt 10 serves as a cushion material, thereby preventing a head part 22a of the occupant 22 from moving in the lateral direction of the vehicle body.