are subjected continuously to the image forming process also when the processing speed is constant a predetermined period of time with the maximum processing capacity can be used in place of the predetermined number of sheets for example in the case that the processing capability is more than 120 sheets hour for the film having the sheet size of 345 u00d7 430 mm when the processing is conducted more than 10 minutes 60 minutes u00d7 20 120 with the maximum processing capacity the calibration means 350 is prohibited calibrating an lut making a new lut for a prescribed period of time as to the prescribed period of time it is not limited to a specified period of time but in the case of the example above it is recommended that the calibration should be stopped after more than one minute according to the present invention this invention provides a heat developing apparatus to obtain an image of stable density without a problem of the abnormal density due to a lut which is formed by conducting calibration based on abnormal data because an lut is not be formed by conducting calibration based on images in the case of obtaining abnormal development results due to instability of the drum temperature which occurs on a heat developing device to conduct heat developing with a surface type heater further according to this invention.