An image forming method comprising: imagewise exposing and thermal developing a photothermographic material using an image recording apparatus, wherein a part of the sheet is exposed and, in parallel with the exposure, development is started on a part of the sheet having been already exposed: wherein the photothermographic material comprises a reducing agent represented by formula (R), and wherein the photosensitive silver halide contains at least two preformed photosensitive silver halides having respective sensitivities different from each other for a light with the same exposure wavelength and a silver iodide content of the photosensitive silver halide is 40% by mole or more.