To mechanically, longitudinally fold two-piece type air bag by providing a pair of horizontal partition plates for pressing an air bag an spreading the same laterally, and providing a pair of chucks for folding the lateral ends of both sides of the air bag and claming the same to be longitudinally folded.

A pair of partition plates 6A, 6B are provided like a long belt parallel to the reciprocating direction of a receiving table 3. The plate surfaces of the partition plates 6A, 6B, are driven by a partition plate horizontally reciprocating mechanism 8 symmetrically moved in the parallel direction to the receiving table 3. Moving plates 13, 13 capable of sliding in the reciprocating manner are provided in symmetrical positions on a guide rail 14, chucks 16A, 16B are provided on both moving plates 13, and the respective chucks 16A, 16B are provided with both of upper and lower clippers 19A, 19B. By such constructed device, two-piece type air bag 20 is positioned on the folding line by the partition plates 6A, 6B, and folded by the clippers 19A, 19B so that the two-piece type air bag can mechanically be folded longitudinally.