An oblique feeding roller and a reference guide are provided on a re-conveying path for conveying a sheet in which an image has been formed on one side by an image forming unit to the image forming unit again. The sheet is obliquely fed by the oblique feeding roller. A side edge of the sheet abuts to a reference surface provided for the reference guide along a sheet conveying direction, thereby correcting the oblique motion and a positional deviation of the sheet. The reference guide is moved to a position according to a length in a width direction of the sheet which is conveyed by a movement of a cam.