PURPOSE: To eliminate the need of dedicated hardwares for respective processings, to reduce a scale and to cope with the addition and change of physical layers in the future by providing a processor inside an LSI and allowing the processor to perform an overhead processing.

CONSTITUTION: Data inputted to a reception frame disassembling part 1 are divided into an overhead and a payload, a prescribed processing is performed to the payload in a cell processing part 2 and it is sent to an ATM layer. The overhead is sent to a memory 3 and the processor 4 reads it, performs a prescribed processing to the various kinds of information and writes it in a status register 5. In the meantime, transmission data are inputted from the ATM layer to a transmission cell processing part 8 in the ATM cell form of 53 bytes, error correction information is put in a cell header and transmission to a frame assembling part 9 is performed. Then, the processor 6 takes out the overhead byte of an overhead memory 7 updated by the information of the register 5 and assembles and transmits a transmission frame. In such a manner, by using the processor and the overhead memory and performing the processings to be performed in the physical layer, the scale of the hardware is reduced.