PURPOSE: To shorten the calculation time by calculating the sum and difference between the address of a data memory means corresponding to a pixel belonging to one of bisected inverse projection regions and the center address stored in a center address memory means and accumulatively calculating the data stored in the sum and difference addresses into each pixel data memory means in the first and second half regions.

CONSTITUTION: An address calculation part 11 calculates the difference between the address of a data memory 15 corresponding to a pixel contained in the upper part region 2a of an image and the center address stored in a center channel register 12 and outputs the difference -Δx. An addition calculator 13 calculates c-Δx, and the data memory 15 inputs the dated stored in the address of c-Δx into the B terminal of an addition calculator 16. The addition calculator 16 addition-calculates the data and stores said data as the new data of the pixel (p) into an upper pixel memory 19. A subtraction calculator 14 performs the calculation c+Δx, and data is added, and the result is stored as the data of the pixel (q) of a new lower pixel memory 20.