It is aimed to provide a folding device and a folding method capable of precisely folding a sheet even in the case of changing a thickness of the sheet or a tension applied to the sheet. A folding device includes a folding mechanism configured to form a first folding line (B1) extending from one widthwise end of a sheet (S1), a second folding line (B2) extending obliquely from the other widthwise end (S12) of the sheet (S1) toward an end of the first folding line (B1) and a third folding line (B3) configured to reverse one widthwise part of the sheet (S1) such that one widthwise part of the sheet (S1) folded along the second folding line (B2) overlaps with another part. The folding mechanism includes a second folding member (11) having a second folding line forming portion (19a) configured to form the second folding line (B2). The second folding member (11) is so attached to a conveying mechanism (2) that the second folding line forming portion (19a) is movable with respect to the formation positions of the first folding line (B1) and the third folding line (B3).
