PURPOSE: To easily form a reversal image by heat-treating diazo photosensitive sheet containing diazonium salt at a part corresponding to image information by the use of the thermal head of a heat sensitive recorder, and then performing a diazo developing step.

CONSTITUTION: Diazo photosensitive sheet is heat treated at a part corresponding to an original image by the thermal head of a heat sensitive recorder. Thus, the part of the sheet corresponding to the image is thermally decomposed at its diazonium salt. Then, the sheet is treated in a diazo developing step. Since the sheet developed in the step is not exposed, the salt is subjected to a coupling reaction to color. However, since the part corresponding to the original image heat treated by the head is thermally decomposed at its salt, a coupling reaction does not occur not to color. As a result, a reversal image uncolored at the image and colored at the background is formed.