An image compression apparatus compresses image data by carrying out a parallel processing for every image pattern made up of a plurality of bits. The apparatus includes a first unit for obtaining a run length of the image data and a repetition number of the run length, a second unit for coding the run length and the repetition number obtained by the first unit, and a third unit for predicting whether or not a code length of a code generated by a word which is to be coded by the second unit exceeds a predetermined threshold value, based on the run length and the repetition number obtained by the first unit. The second unit carries out a pass coding to output the image pattern as it is when the third unit predicts that the code length exceeds the predetermined threshold value.