PURPOSE: To enable prompt output and to enable the images printed on photographic paper to faithfully reproduce the colors and gradations of an original picture with high quality by measuring the transmission density of the image and correcting the fluctuation in individual printing elements according to the measured value.

CONSTITUTION: The colored image is formed by superposing an ink coating surface contg. dyes of an ink sheet 2 and the image receiving layer side of a transparent image receiving sheet 6 on each other and migrating the thermally diffusive dyes in the ink by thermal impressing of a thermal head 4 to the image receiving layer of this transparent image receiving sheet 6 according to image data. The transmitted light quantity of the test image formed on the transparent image receiving layer is measured and the fluctuation in the density of the respective printing elements is determined. The correction values of the respective elements are then set. Then, the fluctuation in the printing densities of the printing elements is calculated from the fluctuation in the transmitted light quantity and is converted to the measured value of the printing energy for correcting the fluctuation in the density. This value is corrected to the impressing energy corresponding to the image signal, by which the exposure mask corrected of the fluctuation in the densities is obtd.