To provide an image processing method for visually clearly displaying a substantially degenerating spot (e.g., floating or peeling spot) in a photographed target.

An image processing method using an infrared thermal image photographed by an infrared digital camera repeats the following steps: step 1 of displaying the infrared thermal image on a display; step 2 of specifying a zone in the infrared thermal image displayed on the display part; step 3 of adding, to a target pixel in the zone specified in the step 2, color information related to the difference between a temperature indicated by temperature data of the target pixel included in the zone and a reference temperature indicated by temperature data of a reference pixel different from the target pixel; and step 4 of displaying the target pixel on the display with the color information.