PURPOSE: To compress dot image data at an angle except for a specified angle without deteriorating compressibility by converting the dot angle of image data on a memory and always converting the dot angle of image data being prediction part input into the specified angle.

CONSTITUTION: Data inputted from an image data input terminal 14 are written into a memory part 17 in a main scanning direction and a sub-scanning direction as against an image area, and image data are accumulated in the memory part 17. Data in prescribed quantity are accumulated and image data accumulated in the memory part 17 are sequentially read in the main scanning direction and the sub-scanning direction by the memory control part 18 with a dot angle designation signal inputted from a dot angle designation signal input terminal 15. The dot angle of image data which are read from the memory part 17 is converted into the specified angle such as 45 degrees, and it is inputted to a prediction part 19, whereby it is converted into prediction data. Prediction data outputted from the prediction part 19 are run-length-converted in an encoding part 20, and it is outputted to an output terminal 16.