An image forming method, including the steps of: charging each surface of a plurality of amorphous-silicon-based photoreceptors; exposing the each surface of the amorphous-silicon-based photoreceptors to form respective color latent images of yellow, magenta, cyan and black; developing the respective color latent images with corresponding color toners to form respective color visible toner images; and transferring the respective color toner images successively to be piled up on a toner image receiving member, wherein the color toner of each color contains grains having volume average grain size (D4) of 3-7 μm and number average primary grain size of 40-800 nm, and the relationship of 1.04≦B/A≦1.4 (6≦A≦30) is satisfied between an exposure size (A μm) in the main scanning direction in the exposing step and a development size (B μm) corresponding to the exposure size (A μm) in the developing step.