To provide a thermal development device capable of providing an image with high diagnostic characteristics in a mammography image requiring a high quality image, and to provide a photothermographic dry imaging material.

The image forming process has the steps of implementing image formation using a thermal development device composed of an exposure section and a thermal developing section for exposing and thermally developing a photothermographic dry imaging material with a support having thereon an image forming layer containing photosensitive silver halide, a light-insensitive organic silver salt, a reducing agent for silver ions and a binder, wherein a surface or both surfaces having the image forming layer are brought into contact with sticky rollers during or before each of the exposure and development so as to make an amount of peel-off static electrification between the photothermographic dry imaging material and the sticky roller from -5 to +5 kV.