PURPOSE: To decrease the number of bits per one picture element without the deterioration in picture quality, by summing a residual luminance signal of pre- processed picture elements to a reading luminance signal of the picture element to be processed.

CONSTITUTION: An m-bit quantizing device 21 receives an analog luminance signal through a signal line 20A and quantizes it to an m-bit digital luminance signal. An n-bit quantizing device 22 uses a residual luminance signal pre-processed before by one and a luminance signal of the device 21 is converted into an n-bit luminance signal. Taking a picture element processed by one as 33, a picture element under processing at present as 34, and a picture processed after by one as 35, the n-bit luminance signal of the picture element 34 is formed by summing a residual luminance signal read out from a memory device 23 before by one to the m-bit luminance signal of the device 21. The residual luminance signal is given to the memory device 23 for the processing of the next picture element 35.