To improve upward development speed of a head protecting portion in an air back, by forming a cylindrical portion from a ventilation hole of a partition sheet to the upper part of the head protecting portion, and by guiding gas in a chest protecting portion through the upper part of the cylindrical portion into the head protecting portion.

Gas for developing a chest protecting portion A of an air back 7 forward intrudes into a cylindrical portion 12 through ventilation holes 10 in a partition sheet 9, and first develops the cylindrical portion 12. Because the cylindrical portion 12 is small in capacity comparing with an entire head protecting portion B, development speed is high. Therefore, the cylindrical portion 12 develops early in the head protecting portion B, the entire head protecting portion B develops upward at a high speed without expansion. Because the head protecting portion B does not expand and is short in width, even when an occupant places his/her arm on an arm rest and a clearance between a door and the occupant is narrow, the head protecting portion certainly intrudes into the clearance. The head protecting portion B expands in the width direction after it intrudes into the clearance, a head portion is protected.