To highly accurately find compression rate for providing target data in a short time in the compression encoding processing of image data.

Scale factors sf1 and sf2 (compression rate) are determined (s2) at the time of performing compression encoding with a thinned image 2 in a much larger image size from the tried result of compression encoding concerning a thinned image 1 (s1). Next, compression encoding is performed by these scale factors sf1 ans sf2 concerning the thinned image 2 (s3) and the correlation of data amount and scale factors sf is approximated from the result of compression encoding with these scale factors sf1 and sf2. Then, a scale factor sf0 is determined corresponding to a target data amount from these approximated characteristics (s4) and concerning an object image provided by main scanning, compression encoding is performed by this scale factor sf0 (s5).