A creasing device, that forms a crease on a sheet, includes: a first member, extending in a direction perpendicular to a sheet conveying direction, on which a convex blade is formed; a second member on which a groove-like concave blade is formed such that the convex blade can be fitted into the concave blade by interposing the sheet therebetween; a drive unit that causes the first and the second members to interpose, therebetween, the sheet to form a crease on the sheet; a sheet-information acquiring unit that acquires first sheet information of the sheet to be creased; an adjusting unit that adjusts a pressing force exerted by the drive unit; and a control unit that sets the pressing force to an optimum pressing force for the sheet and that causes the drive unit to drive the first and the second members for creasing the sheet at the optimum pressing force.