An image decoding apparatus decodes transmitted compressed image data. The apparatus includes a receiving buffer for temporarily storing the compressed image data, a decoder for reading compressed image data stored in the receiving buffer corresponding to proceeding conditions of a decoding process and decoding image data of one frame within a period equal to the frame period of the image or within a period shorter than such frame period, and an eliminating circuit which is providing in the receiving buffer to supply the compressed image data to the receiving buffer after eliminating invalid code to prevent underflow of data in a send buffer provided in an image coding apparatus.