PROBLEM TO BE SOLVED: To provide a device and method for folding and superposing a planar flexible member, capable of reducing a moving range of a belt conveyor and a conveyor roller of a carrying device, achieving a compact folding-up superposing device with a simple structure, and accurately adjusting a joint of the edge of the planar flexible member, without using an excessive member such as an insertion member. SOLUTION: An upper carrying passage is formed of a first belt conveyor 1 and a second belt conveyor 2, and a mat is guided to a third belt conveyor 5 under its upper carrying passage by a guide member 6 via a clearance 7 under its upper carrying passage. One mat is carried by straddling the clearance 7, and is put in a folding-up state by being sent into below the clearance 7 from a mat intermediate part, or two mats are sent into below both sides of the clearance 7 by oppositely transferring via the clearance 7, and mutual surfaces are put in a superposed superposition state, and are transferred onto the third conveyor. COPYRIGHT: (C)2010,JPO&INPIT