1. An image forming apparatus comprising: an original placement portion: reference original means provided at an appropriate position outside of the original placement portion; scanning means capable of selectively scanning either the original placement portion or the reference original means; image forming means for forming an image based on a result of the scanning means; selection means for selecting the scanning of the original placement portion or the scanning of the reference original means; and reading means for reading an image of the reference original means formed by the image forming means to evaluate a performance of the image forming means; the image forming means including: a photosensitive body for producing a latent image based on a result of the scanning means; developing means for developing the latent image to a toner image; transferring means for transferring the toner image onto copy paper; and conveying means for conveying copy paper through the transferring means; and the reading means includes a detector provided on a downstream side of the transferring means for detecting the leading edge of copy paper carrying a toner image of the reference original means and the toner image: the image forming apparatus further comprising: memory means for storing a reference timing difference between the leading edge of copy paper and the reference original image; timing difference measuring means for measuring a timing difference between a detection of the leading edge of copy paper and a detection of the reference original image; offset value calculating means for calculating an offset value between the reference timing difference and the measured timing difference; and control means for controlling the conveying means to convey copy paper to the transferring means at such a timing as to correct an offset value calculated by the offset value calculating means.