PURPOSE: To obtain the gradation of contrast from the original of the low contrast by providing a level converting means to input the output of a quantizing means and to output it while converting to a prescribed correspondent relation and a control means to set the correspondent relation of conversion at the level converting means corresponding to the range of a quantizing density level to be obtained from a detecting means.

CONSTITUTION: For multilevel quantized data, appearance frequency (density histogram) for each density is counted by an adder 7 and stored in a histogram memory 8. After this prescanning, a CPU 11 outputs a conversion table to a conversion table memory 10 by referring the contents of the histogram memory 8. Next, a switch 6 is changed over to a transformer 9 side and scanning is executed for outputting picture signals. In such a case, the multilevel quantized data are inputted from an A/D conversion part 2 to the transformer 9 and according to the conversion table in the conversion table memory 10, level conversion is executed. Then, the data are outputted after executing a gradation processing such as a dither processing, etc., by a gradation processing part 5.