PURPOSE: To provide a device for rapidly and correctly fold a plurality of forms along perforations, continuously sent.

CONSTITUTION: Forms P delivered by delivering rolls 11, 12 are successively folded to a discharge belt 20 side by rotary arms 31, 32 which are alternately rotated. The rotary arms 31, 32 and the guide surface of guide members 41, 42 which are alternately arranged form the folding surface. When the folding operation of the forms is completed, the guide members 41, 42 are rotated to load the folded forms on the discharge belt, and the new guide surface of the guide members 41, 42 becomes the folded surface to start the folding operation of the forms to be delivered next. The folding device is connected between a cutting device and an enveloping device in an envelope preparing system, and synchronized with these device to be drive-controlled.