To provide an area gradation image forming method by which a high-quality proof image is stably obtained with neither displacement in exposure nor peeling from an external surface drum and with no problem on drying property and transportability in equipment.

In the area gradation image forming method in which, after storage at ≤0°C for ≥100 days, a silver halide photosensitive material is wound on the external surface drum, exposed and developed to form an area gradation image, the silver halide photosensitive material has a gelatin-containing layer containing a vinylsulfone film hardening agent on a side opposite to a silver halide emulsion layer, drying is carried out after setting after coating, and the material is heat-treated after the storage at ≤0°C and before the exposure in such a way that tf value defined by the expression: tf=∫exp(-K/T)dt becomes ≥4×10-11.