PURPOSE: To execute an image reduction without degrading the quality of a reduced image by means of simple image processing prevented from the increment of cost.

CONSTITUTION: This image reduction processing system is provided with a dot information judging processing means 1 for judging the number of black dots in image data, thinning reduction processing means 2 for executing thinning processing and an n-dot OR reduction processing means 3. When there are black dot data ≥2 dots at the time of checking the number of black dots out of 2n dots in the image data in the main scanning/subscanning direction, 1/n thinning reduction processing is executed, and when the black dot data of only one dot exist, n-dot OR reduction processing is executed.