The invention provides a creasing device and an image forming system. The creasing device includes: a first member on which a linear convex-edged blade is formed in a direction perpendicular to a sheet conveying direction; a second member on which a concave-edged blade pairing with the convex-edged blade is formed; and a drive unit that drives the first and second members to relatively come close to and away from each other, thereby making a crease on the sheet, wherein in a standby state, the drive unit keeps faces of the first member and second member not parallel; and in making a crease on the sheet, causes the convex-edged blade and the concave-edged blade to have point contact with each other via the sheet held therebetween so as to initiate a creasing movement by a rotation movement thereof.