The present invention provides a sheet folding device and a carton former. In the present invention, a folder/gluer unit (61) is provided with: an upper conveyance belt (81) that conveys a corrugated cardboard sheet in a direction (D) of conveyance; a pair of forming belts (84) that contact a folding surface formed by both ends in the widthwise direction perpendicular to the direction (D) of conveyance of the corrugated cardboard sheet being folded and that fold both ends in the widthwise direction of the corrugated cardboard sheet; a pair of folding bars (85) that are provided along the direction (D) of conveyance and contact the folding surface at both ends in the widthwise direction of the folded corrugated cardboard sheet, guiding both ends in the widthwise direction of the corrugated cardboard sheet; a moving mechanism (86) that causes the pair of folding bars (85) to move in the widthwise direction and the vertical direction; and a control device (100) that adjusts the position of the folding bars (85) by controlling the moving mechanism (86).