In a construction for forming images on the both surfaces of a sheet, a speed controlling device for effecting the conveyance control of the sheet between a transfer device and a fixing device controls, when the image is to be formed on a first surface, the sheet conveying speed between the transfer device and the fixing device on the basis of loop detection by a loop detecting sensor provided between the transfer device and the fixing device so that a loop formed in the sheet may be maintained within a predetermined range. Also, when the image is to be formed on a second surface, the speed controlling device controls the sheet conveying speed between the transfer device and the fixing device on the basis of the average sheet conveying speed between the transfer device and the fixing device when the image has been formed on the first surface.