A paper correction apparatus B corrects a curl of a sheet S by passing the sheet S through curved paper conveying routes CCP1 and CCP2 whose curving amounts can be changed. The paper correction apparatus B is provided with a control unit B3 which sets the curving amounts of the curved paper conveying routes CCP1 and CCP2, and changes a sheet interval in accordance with the curving amounts which are set. By this configuration, it is possible to inhibit the productivity from being reduced due to paper correction during performing post-printing processes in a finisher which is located in the downstream side of the paper correction apparatus B.