A creasing device for creasing sheets on a per-sheet basis, the creasing device includes: a first member extending perpendicularly to a sheet conveying direction and including a male blade, which has a convex cross section; a second member extending perpendicularly to the sheet conveying direction and including a grooved female blade, into which the male blade to be fitted with a sheet between the female blade and the male blade; and a drive unit that brings the first member and the second member relatively into and out of contact with each other to cause a sheet stopped at a predetermined position to be pinched between the first member and the second member and creased. An edge portion of any one of the first member and the second member has an arcuate shape.