PURPOSE: To widely decrease the storage capacity of a digitized series of image data in the image data compressing/expanding system.

CONSTITUTION: Plural digitized image data are inputted, and on an image which becomes an object, the attention is paid to the feature of the data between respective pixels to constitute the image. Further, an image data compressing part to pay attention to the feature such as similarity and output the compressed image data for which the data quantity is decreased and an image data expanding part to input the compressed image data and output the image data restored to the original condition are equipped. The storage capacity necessary to store a series of plural digital image data can be widely decreased.