Multicolor image forming method and apparatus therefor

An image forming apparatus capable of producing pure and clear-cut bicolor images stably over a long period of time. After a photoconductive element has been uniformly charged by a first charger, a first latent image is formed on the element by first exposure. The first latent image is developed by a black toner stored in a first developing unit. Subsequently, after the photoconductive element has been charged by a second charger implemented by a scorotron charger, a second latent image is formed on the element by second exposure. The second latent image is developed by a red toner stored in a second developing unit by reversal development. The resulting two toner images are transferred to a recording medium. The grid voltage of the second charger is selected to be lower than the potential deposited on the photoconductive element by the first charger. As a result, the portion of the image carrier where the black toner is deposited by the second developing unit has a lower potential than the portion where the black toner is not deposited. This increases the difference between the potential of the portion where the black toner is not deposited and the bias potential for development, thereby insuring a margin against the contamination of the background. At the same time, the difference between the potential of the portion where the black toner is deposited and the bias potential for development is reduced to prevent the black toner from flying away from the photoconductive element.