An accumulation unit accumulates, in a 1-line element corresponding to the main scanning direction, an electrical signal prepared by photoelectrically converting optical image information obtained by scanning an original image to be formed with a carriage in the subscanning direction. A magnification setting unit sets the copy magnification. A speed change unit changes the moving speed of the carriage in accordance with the copy magnification, and when the set copy magnification is equal to or smaller than a certain magnification in reduction, changes the carriage moving speed to 1/n (n: a natural number larger than 1) from the relationship between the current magnification and the carriage moving speed. A preprocessing unit converts the electrical signal into a digital signal, has a line memory for storing 1-line input image data in the main scanning direction, performs arithmetic processing for multilevel n-line data in the main scanning direction to change the subscanning magnification in reduction, and outputs 1-line output image data in the main scanning direction. An image data processing unit performs predetermined image data processing including processing of receiving for each pixel the 1-line output image data in the main scanning direction output from the preprocessing unit, and converting the image data into binary data.