To effectively use a line by extracting a null packet from an input packet string and multiplexing low speed data.

A null packet header detection part 2 monitors the header part of the input packet string 5 and detects the header of the null packet. The stored position of the null packet in a memory 3 is transmitted to a low- speed data packet insertion part 4. The low-speed data packet insertion part 4 rewrites the null packet in the designated storage position by a low-speed data packet. Since data is inserted into a position which is the originally null packet, low-speed data of a PN code can be transmitted without affecting the other packets at all.