r C, D; in the case of mode A or B, it is checked in step S96 whether the mode is A or B. If the mode A is decided, the initializing flag IFLG is checked in step S34, and if the initializing flag IFLG is 1 which indicates an initializing block, the program proceeds to step S113, in which the 4???4 block is decoded as the average gray level code through utilization of the gray level codes read out