To provide a thermal development method and a thermal development device for a photothermographic dry imaging material where an image having high diagnostic characteristics can be provided even if the size of a cooling section is relatively made small due to compacting of the device.

At the time when a latent image is formed on the exposed part in a photothermographic dry imaging material having a light-insensitive surface on one side and having a light-sensitive surface on the other side, and is thermally developed by the thermal development device, the thermal development device includes: a thermal development section; a cooling section arranged on the downstream side of the thermal development section and cooling and conveying the photothermographic material conveyed from the thermal development section, also, the ratio of the length of the cooling section to the thermal development section is ≤1.5 to the length of the thermal development section, and further, a cooling rate for the light-insensitive surface of the photothermographic dry imaging material is faster than the cooling rate for the light-sensitive surface of the photothermographic dry imaging material.