An image forming apparatus, which realizes a buffering function which enables post-processing without decreasing the productivity and increasing the size. At least one image is formed on a sheet. A double-sided sheet conveying path is provided, which the sheet passes when being conveyed back to the image forming unit again to form an image on an opposite side surface of the sheet after an image is formed on one side surface of the sheet. A retaining member is provided on the double-sided sheet conveying path to retain the sheet on the double-sided sheet conveying path. A post-processing device carries out post-processing on sheets on which images have been formed by the image forming unit. A CPU circuit section, which controls conveyance of the sheet, provides control to convey the sheet on which the at least one image has been formed by the image forming unit to the double-sided sheet conveying path, cause the retaining member to temporarily retain the sheet on the double-sided sheet conveying path, and then convey the sheet to the post-processing device.