A saddle unit provided with a fold-enhancing unit for performing fold enhancement of sheets of paper folded in the middle. The fold-enhancing unit includes a fold-enhancing roller mechanism which performs the fold-enhancing operation in the direction orthogonal to the sheet carrying direction. The fold-enhancing roller mechanism includes a fold-enhancing roller pair that pinches and presses a bundle of sheets at a nip. The roller guide pair provided in the fold-enhancing roller mechanism guides the sheets of paper to the nip of the fold-enhancing roller pair when enhancing the fold of the sheets of paper. The saddle unit changes one or both of the number of times and the velocity of the reciprocating movement according to one or both of the number of sheets of paper and the quality mode which is entered and set by the user from the operation panel of an image forming apparatus or job-set by the personal computer by a user.