PURPOSE: To reduce the whole package capacity by extending channels by introducing high-speed memories on occasion by making use of memories packaged initially, by constituting buffer memories by combining low-speed memories and high-speed memories.

CONSTITUTION: Data of a time slot on highway #1 is applied to block A of buffer memory 6, and data on highways #2 to #4 are to high-speed buffer memory 14 by way of high-speed multiplexer 12. Further, selector 11 supplied with the output of memory 6 and selector 11 connected to demultiplexer 15 supplied with the output of memory 14 are provided, and address access to memory block A and memory 14 is attained by holding memories 7. Memory block A on a channel system of this constitution is composed of low-speed memories 6 connected together in parallel and high-speed memory 14 is connected to this block A to constitute a buffer, thereby simplifying memory extension at the time of an increase in channel capacity.