A sheet folding device and method, and a box-making machine, wherein are provided a molding belt (108) for moving on a center side in the width direction of a cardboard sheet (S) toward a downstream side in a conveyance direction (D) of the cardboard sheet (S) and thereby pressing and folding both end parts in the width direction of the cardboard sheet (S) from the outside, and a folding roller group (102) formed by folding rollers (121, 122, 123, 124, 125, 126) for contacting insides of fold parts on both sides in the width direction of the cardboard sheet (S) more toward the center in the width direction of the cardboard sheet (S) than the molding belt (108), the folding rollers (121, 122, 123, 124, 126, 126) being arranged further upstream in the conveyance direction (D) of the cardboard sheet (S) than a 90-degree folding position of the cardboard sheet (S).