To provide a communication unit that codes voice or other signals into digital information and transmits the information to a data communication network such as the Internet.

A coding rate selector 13 selects a coding rate of an output of an analog/digital converter 12. A voice-coder 14 compression-encodes output of the A/D converter 12 according to selected coding rate. A frame aggregation device 15 receives an output of the voice-coder 14 to arrange coding frames and to generate an IP frame with a prescribed time length for the transmission of the arranged frame onto the Internet 1 and outputs the IP frame. The received IP frame is disassembled into coding frames. A fluctuation absorption buffer 23 stores the coding frame for a prescribed time and outputs the frame to a decoder. Since it is not required to transmit an IP packet consisting of coding frames at a small coding rate with a header added thereto, the overhead by the header can be reduced. The fluctuation absorption buffer 23 absorbs a transmission delay.