PURPOSE: To improve the efficiency of memory capacity by varying the gradation of an image at the processing of an objective image so that data can be stored in an image memory.

CONSTITUTION: When an objective image does not require high gradation, an arithmetic unit 4A selects an A/D converter 2 (2.2) having a 4-bit gradation output. Since the gradation of the selected data 15 is 4 bits though that of the picture memory is 8 bits, a data conversion indicating signal 17 is outputted to an input data conversion part 18 so that data are to be inputted to an area 1 in the image memory 3 and the input data conversion part 18 outputs data to the area 1 of the image memory 13. If the arithmetic unit 4A selects an A/D converter 3 (2.3) having a 2-bit gradation output form the succeeding objective image, the data are inputted to the area 2 of the image memory 3, and when selecting the A/D converter 3 (2.3) continuously, the data are inputted to an area 3 of the image memory 3.