To provide a paper sheet folding mechanism for folding a rectangular paper sheet for back lining into a flat tubular shape.

The paper sheet folding mechanism includes: a conveyance part 14a conveying a rectangular paper sheet 4 along a conveyance path X by sucking onto a sucking belt 43; a shape-forming ruler 14b arranged opposite to the sucking belt 43 along the conveyance path X for sandwiching a central portion of the rectangular paper sheet 4 between the shape-forming ruler 14b and the sucking belt 43; a raised part 14c arranged upstream of the conveyance path X in the shape-forming ruler 14b and folding side parts on both sides of the central portion of the rectangular paper sheet 4 sandwiched between the sucking belt 43 and the shape-forming ruler 14b to raise the side parts from the sucking belt 43; and a pair of folding parts 14d, 14e arranged on both sides of downstream of the conveyance path X in the shape-forming ruler 14b and brought into contact with the raised side parts, and forming the rectangular paper sheet 4 to have a flat tubular shape by rubbing the side parts to the central side and the downstream side of the shape-forming ruler 14b and folding them on the shape-forming ruler 14b.