A creasing apparatus forming a crease on sheets one by one includes: a first member having a linear convex blade formed in a direction perpendicular to a sheet conveying direction; a second member having a concave blade being paired with the convex blade; and a drive unit that moves the first and second members so as to cause the convex blade and the concave blade to form a crease on a sheet stopped at a predetermined position. The first member forms the convex blade with first comb and second comb and the second member forms the concave blade with third comb and fourth comb that advance and retract relative to each other. The drive unit selects advanced and retracted positions of the first comb and second comb and advanced and retracted positions of the third comb and fourth comb so as to perform a creasing process or a perforating process.