PURPOSE: To improve folded distortion by forecasting the generation of folded distortion in each block.

CONSTITUTION: An AC component storing memory 32 stores only an AC component in a signal for one block from a 2D-DCT part 31. At the time of ending the conversion of the part 31, an AC component energy calculating part 33 calculates the sum of energy in the AC component for one block by using the information of the memory 32. When the energy sum of the AC component calculated by the part 33 exceeds a prescribed value, a folded distortion forecasting part 34 generates 1-bit informatin in each block. A forecasted result storing part 28 stores information obtained by an AC component forecast inspecting part 27 correspondingly to a block position. An image processing part 29 executes partial image processing in each block based upon the information stored in the part 28. In a part where the generation of folded distortion is forecasted, the signal level of a picture element generating a pattern whose property is sharply different from that of peripheral picture elements due to folded distortion can be corrected based upon relaton with the peripheral picture elements.