A device including: a main folding mechanism for folding a diaper body in two at the crotch portion by rotating first and second main pads having first and second holding surfaces, respectively; and a sub-folding mechanism for folding the flap by rotating a side pad having a sub-holding surface for holding the flap with respect to the first main pad, wherein; a cut-out is formed at a position corresponding to the attachment portion of the second main pad, wherein the cut-out does not allow the attachment portion to be held by the second main pad but instead expose the attachment portion; with the first and second holding surfaces being closed while opposing each other and with the main body folded in two, the cut-out allows the flap to be attached to the attachment portion; and the side pad rotates from a position where it is open while lying next to the first main pad in the girth direction to a position where the sub-holding surface is closed while opposing the first holding surface, fitting into the cut-out.