To provide a folding method for a side collision airbag capable of shortening the completion time for developing an airbag compared to conventional methods, and a folding device for a side collision airbag used for the folding method.

This folding method includes: a first process for vertically folding at least one of the upper portion 12a and the lower portion 12b of a developing and inflating part 12 into the developing and inflating part 12; and a second process for cactus-folding the developing and inflating part 12 in the vehicle longitudinal direction. This enables gas introduced into the airbag 10 to roughly simultaneously touch all the inside surface of the developing and inflating part 12 when the developing and inflating part 12 develops. In addition, this enables the developing and inflating part 12 to almost concurrently develop both in the vehicle longitudinal direction and in the vertical direction when the developing and inflating part 12 develops. Accordingly, this can shorten the completion time for developing the airbag 10 compared to when the developing and inflating part develops in either of the vehicle longitudinal direction or the vertical direction before developing in the other direction.