A method for assembling a height-adjustable box, the method comprising: supplying a base sheet on which at least one product is to be placed and a cover sheet which is to be placed on the base sheet; forming first fold lines (a) on the base sheet and the cover sheet; measuring a height of the product; forming second fold lines (b) on each of the base sheet and the cover sheet after forming the first fold lines (a), so as to be spaced from the respective first fold lines (a), such that the base sheet includes a bottom sheet portion defined between the first fold lines (a) of the base sheet; side sheet portions each defined between one of the first fold lines (a) of the base sheet and the corresponding one of the second fold lines (b) of the base sheet; and folded sheet portions each extending from one of the second fold lines (b) of the base sheet to a corresponding edge of the base sheet, and such that the cover sheet includes a top sheet portion defined between the first fold lines (a) of the cover sheet; side sheet portions each defined between one of the first fold lines (a) of the cover sheet and the corresponding one of the second fold lines (b) of the cover sheet; and folded sheet portions each extending from one of the second fold lines (b) of the cover sheet to a corresponding edge of the cover sheet, wherein a space for the side sheet portions (12, 22) between the first fold lines (a) and the corresponding one of the second fold lines (b) is adjusted based on a value of the measured height of the product; bending the base sheet only along the first fold lines (a) and the second fold lines (b) in a same direction such that the base sheet is rolled in an upwardly direction; placing the cover sheet on the base sheet with the base sheet and the cover sheet arranged so that the base sheet crosses the cover sheet; bending the cover sheet along the first fold lines (a) and the second fold lines (b) in a same direction such that the cover sheet is rolled in a downward direction; and bonding the folded sheet portions of the base sheet to the top sheet portion of the cover sheet, and bonding the folded sheet portions of the cover sheet to the bottom sheet portion of the base sheet. supplying a base sheet on which at least one product is to be placed and a cover sheet which is to be placed on the base sheet; forming first fold lines (a) on the base sheet and the cover sheet; measuring a height of the product; forming second fold lines (b) on each of the base sheet and the cover sheet after forming the first fold lines (a), so as to be spaced from the respective first fold lines (a), such that the base sheet includes a bottom sheet portion defined between the first fold lines (a) of the base sheet; side sheet portions each defined between one of the first fold lines (a) of the base sheet and the corresponding one of the second fold lines (b) of the base sheet; and folded sheet portions each extending from one of the second fold lines (b) of the base sheet to a corresponding edge of the base sheet, and such that the cover sheet includes a top sheet portion defined between the first fold lines (a) of the cover sheet; side sheet portions each defined between one of the first fold lines (a) of the cover sheet and the corresponding one of the second fold lines (b) of the cover sheet; and folded sheet portions each extending from one of the second fold lines (b) of the cover sheet to a corresponding edge of the cover sheet, wherein a space for the side sheet portions (12, 22) between the first fold lines (a) and the corresponding one of the second fold lines (b) is adjusted based on a value of the measured height of the product; bending the base sheet only along the first fold lines (a) and the second fold lines (b) in a same direction such that the base sheet is rolled in an upwardly direction; placing the cover sheet on the base sheet with the base sheet and the cover sheet arranged so that the base sheet crosses the cover sheet; bending the cover sheet along the first fold lines (a) and the second fold lines (b) in a same direction such that the cover sheet is rolled in a downward direction; and bonding the folded sheet portions of the base sheet to the top sheet portion of the cover sheet, and bonding the folded sheet portions of the cover sheet to the bottom sheet portion of the base sheet.