Vehicle-use side air bag module manufacturing method and vehicle-use side air bag device

[Problem] To provide a vehicle-use side air bag module manufacturing method and a vehicle-use side air bag device that enable the stored dimensions of a side air bag, which has an inflator accommodating region and which is folded in an stored state, to be short lengthwise and to be made small for the diameter dimensions. [Solution] In a side air bag 2, formed is an inflator accommodating region Q for accommodating an inflator on the rear side in the vehicle front-rear direction. The present invention includes the following: a folding step in which, according to a fold line F which crosses the side air bag from the vehicle rear to the vehicle front, an upper region R which is above the fold line is folded so as to overlap a lower region S which is below the fold line, thereby setting for the side air bag a fold-up region T for folding up the side air bag into a conical body shape the upper part of which is tapered and the lower part of which is widened toward the end; and a fold-up step in which a fold-up center U of at least one position is set on a fold line which forms the upper edge of the fold-up region, the fold-up center serving as a reference for beginning fold up, and the fold-up region is folded up towards the inflator accommodating region to form a folded-up part.