PURPOSE: To perform easily and accurately a temporary folding work to fold the top of a carton into a specified shape along folding grooves by a method wherein the carton is formed into a specified shape of box, and at the same time, a folding jig for which a specified guide protuberance is provided is elevated to the carton.

CONSTITUTION: In performing the temporary folding work, a second hydraulic cylinder 43 for an elevating means 21 is extended, and then, a core metal 22 is lowered by a slide shaft 42, and inserted in a carton 1. Then, a first hydraulic cylinder 35 is extended, and a folding jig 20 is lowered by a piston rod 35a to cover the carton 1. Then, the carton 1 is constrained by a square cylindrical lower part 23 of the folding jig 20, and a triangle shaped part is folded inside of a flat surface part 12 by a lower surface edge 45 of a guide protuberance 32. At the same time, a sealing part 27 is pushed inside by a tilted surface 46 of an upper part 24. Then, accompanying with the descent of the folding jig 20, the sealing parts 27 are put together in a slit 26, and at the same time, the flat surface part 12 is pressed by the tilted surface 46 to complete the temporary folding.