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.

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 layer containing a vinylsulfone film hardening agent, 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, wherein t represents time (min); K represents a constant which is 1.006×104; and T represents the temperature (K) of the environment in which the silver halide photosensitive material is put.