1. An image forming apparatus comprising: an image bearing member capable of bearing a toner image; a rotatable transfer member for transferring the toner image from said image bearing member onto a recording material while nip-feeding the recording material between itself and said image bearing member at a transfer portion; conveying means for conveying the recording material having the toner image on a first side thereof to transfer the toner image onto a second side of the recording material; control means for controlling an operation for forming a test toner image on said image bearing member in an interval region between consecutive output images during continuous image formation for successively forming the output images; detecting means for detecting the test image, wherein said control means permits the formation of the test image, when next one of the consecutive output images is to be transferred onto a first side of the recording material, and said control means is capable of preventing the formation of the test image, when next one of the consecutive output images is to be transferred onto a second side of the recording material.