PURPOSE: To contrive a reduction in the size of total boxing device and in the boxing operation costs by simplifying the method for forming cartons and the structure of the carton folding part of a boxing device.

CONSTITUTION: An insertion body 2 is inserted between a top panel 22 and a bottom panel 23 of a flat carton forming sheet 21 for use in forming the carton to raise the left and right side pieces 24 and 25 of the carton forming sheet 21 in order to form opening parts 31 at the front and rear of the carton. After the left and right side pieces 24 and 25 are kept in a standing position with retaining plates held thereagainst, the bending pieces 28 and 29 formed extending from the left and right side pieces 24 and 25 are bent to the side of the opening part 31 by pressing a bending instrument thereagainst.