In order to assemble a box for sending products in which the height of the box is freely (steplessly) changed according to the size of each of the products, a base sheet (1) and a cover sheet (2) are supplied, the height of a product which is placed on a bottom sheet portion (11) of the base sheet (1) is measured, second fold lines (b) are formed on the base sheet (1) and the cover sheet (2) such that the second fold lines (b) are spaced from respective first fold lines (a) formed on the respective side portions of the a bottom sheet portion (11) and a top sheet portion (21), wherein the space between each of the first fold lines (a) and the corresponding one of the second fold lines (b) is adjusted based on the value of the measured height of the product, wherein the spaces between the respective first fold lines (a) and the second fold lines (b) of the base sheet (1) are side sheet portions (12) and the spaces between the respective first fold lines (a) and the second fold lines (b) of the cover sheet (2) are side sheet portions (22), wherein the spaces between the respective second fold lines (b) of the base sheet (1) and the edges of the base sheet (1) are folded sheet portions (13) and the spaces between the respective second fold lines (b) of the cover sheet (2) and the edges of the cover sheet (2) are folded sheet portions (23), each of the side sheet portions (12) and the folded sheet portions (13) of the base sheet (1) is folded in the same direction, the base sheet (1) and the cover sheet (2) are arranged so that the base sheet (1) crosses the cover sheet (2), the cover sheet (2) is placed on the base sheet (1) so that the top sheet portion (21) is placed on the folded sheet portions (13) of the base sheet (1), and each of the side sheet portions (22) and the folded sheet portions (23) of the cover sheet (2) is folded in the same direction to bond the folded sheet portions (23) of the cover sheet (23) to the bottom sheet (11).