Folding device and folding method using same

A folding device for folding a sheet in a width direction such that one widthwise part of the sheet overlaps with another part while conveying the sheet in a longitudinal direction thereof, comprising:

• a conveying mechanism configured to convey the sheet in the longitudinal direction;
• and
• a folding mechanism configured to form a first folding line, a second folding line and a third folding line of the sheet being conveyed by the conveying mechanism, the first folding line extending in the width direction from one widthwise end to a widthwise intermediate part of the sheet, the second folding line extending obliquely to the longitudinal direction from the other widthwise end of the sheet on a side upstream of the first folding line in a conveying direction toward an end of the first folding line and the third folding line being configured to reverse one widthwise part of the sheet such that one widthwise part of the sheet folded along the second folding line overlaps with another part on a side downstream of the first folding line in the conveying direction,
• wherein:
• the folding mechanism includes a second folding member having a second folding line forming portion configured to form the second folding line of the sheet;
• and
• the second folding member is so attached to the conveying mechanism that the second folding line forming portion is movable with respect to the formation positions of the first and third folding lines of the sheet.