PURPOSE: To compensate a storage error of an image data in a short time with a low cost, by reading the image data out of an image memory with each line, and compensating and outputting the image data having the storage error for each line.

CONSTITUTION: A check code is provided to image data with each line, and the image data are stored temporarily in the 1st line buffer with each line from an image memory 1. In this case, the 2nd line buffer 3 stores temporarily the 1-line preceding image data. The buffer 2 inspects the check code of the image data of each line, and a switch circuit 4 selects and outputs the image data containing no error. If an error is detected, the circuit 4 selects and outputs the 1-line preceding image data that is stored in the buffer 3 and generally has the same value.