PURPOSE: To provide the compressor and expander for image data in which image data are compressed at a high compression rate without production of a noise in a transparent picture element in a transparent image.

CONSTITUTION: The compressor and expander has a compression data generating means 5 applying irreversible coding to a transparent digital image expressed by using a transparent color expressed by prescribed picture element data and a display color expressed by using other picture element data with respect to a digital image to generate image compression data, an error correction data generating means 8 generating transparent data representing it that a picture element is a transparent when the picture element forming the transparent picture is transparent, and also an expansion data generating means 6 decoding the image compression data to generate picture element data, and a composing means 10 setting picture element data in a transparent color as to picture elements representing a transparent color and setting picture element data of a picture element corresponding to the picture element not representing the transparent color generated by the expansion data generating means.