PURPOSE: To increase information at the time of transforming input image data into image data having more quantization bits.

CONSTITUTION: A blocking circuit 32 divides input image data (8 bits) inputted from an input terminal 31 into plural blocks. A detection circuit 33 detects the maximum and minimum values of picture element levels in each block and a subtractor 37 finds out a level change in the block. The level change is supplied to a comparator circuit 38 and compared with a threshold Th inputted from a terminal 39. A class code generating circuit 36 generates a class code (c) from the 8-bit data, a bit pattern corresponding to the code (c) is read out from a data memory 9 and added to the lowest bit of the 8 bits through a lower bit adding circuit 35. A selector 40 outputs one of outputs (10-bit data) from a '00' adding circuit 34 and the circuit 35 in response to the compared result of the circuit 38.