The invention provides an imaging system wherein toners of two or more colors are used to form toner images and the toner images are successively used to form a color image on a recording material. The imaging system can form an image with high transfer efficiency. An electrostatic latent image is formed on an image carrier. Using developing units for two or more colors, images are formed. Then, the images are successively transferred onto an intermediate transfer medium at a transfer voltage fed from a constant-voltage power supply. The developing units are located such that development occurs in descending toner work function order.