An image forming apparatus comprises a fixing unit for fixing toner image onto a sheet on which the toner image is formed, and a redelivery path for delivering the fixed sheet ejected from the fixing unit to the image forming unit again. A curved path receives the fixed sheet ejected from the fixing unit, reverses the sheet, and delivers the sheet to the redelivery path again. The redelivery path includes a horizontal-registration detection sensor for detecting the edge of the sheet in the width direction and a color density sensor for detecting the color density and color of an image formed on the sheet. A curl-corrector is disposed downstream of the curved path to straighten the curled sheet and then send the sheet to the horizontal-registration detection sensor and the density sensor.