PURPOSE: To convert a frame format by using a SRAM even when a write request and a read request take place in duplicate by providing the SRAM used for data write and read when the format of a transmission frame is converted.

CONSTITUTION: The converter is made up of a serial/parallel conversion circuit (S/P) 1a, a frame format conversion SRAM 2a, a parallel/serial conversion circuit (P/S) 3a and a control circuit 4a. The SRAM 2a has a function of converting a frame format of reception data into H12. Then each period of a transmission clock of a conversion destination is divided into halves, the first half is set for a write period and the latter half is set for a read period and when a data write request takes place, the data are converted in the SRAM 2a at a first half of each of transmission clock period of a conversion destination. On the other hand, when a data read request takes place, the data are read from the SRAM 2a at a latter half of each of transmission clock period of a conversion destination.