PURPOSE: To uniform the temperature distribution of a sheet for image formation and to eliminate an irregularity in density after development by controlling the temperature of the center part of the outer periphery of the heat drum to temperature higher than the temperature of both end parts before the start of heat development processing.

CONSTITUTION: The temperature of the heat drum 34 before the start of processing is controlled into a specific temperature distribution so that heat conducted actually to a photosensitive material becomes uniform. This specific temperature distribution is found from an experiment result and the center part and both end parts are given the same temperature characteristics by generating a temperature difference equal to the mean temperature distribution difference between both characteristics so that the center of the heat drum 34 is a specific quantity larger than both end parts. For example, both end parts of the heat drum 34 are heated by subordinate halogen lamps 46 to limit the quantity of heat radiation. Consequently, the irregularity in image density generated at the time of heat development processing is eliminated and the density distribution becomes uniform over the entire area of a photosensitive material.