PURPOSE: To reduce power consumption by speeding up change-point detection by scanning change points in blocks of picture element.

CONSTITUTION: The detection of change points is started once a flip-flop FF21 is set with a change-point detection start signal ST. At the time, a request signal C1 for the change-point detection of a next picture element block is generated as the output of a logic array 17 and a latch circuit 19 latches the value of a counter circuit 18 while an FF15 latches the picture element data of the picture element block address of a coding line and the value of the most significant digit bit of the last picture element block. Then, a detection request signal C2 for change points sets the FF21 for change-point addresses A1 and A2 among addresses of the picture block and the data of a color signal at a change point, and at the same time, a latch circuit 20 is set, thus completing the detection.