PURPOSE: To improve the transmission error detection rate to a long packet and to improve the transmission efficiency to a short packet by providing a transmission circuit sending a transmission frame added with a CRC code having a length in response to a packet length and a reception circuit applying CRC check.

CONSTITUTION: A packet kind decision circuit 12, according to the result of packet kind decision outputs a packet kind command signal 31 representing the packed kind and a selection signal 32 (logical '1' for a long packet and logical '0' for a short packet). The field for packet kind information S is pre pared for the frame. A CRC generation circuit 16 is activated when the selection signal 32 is logical '1', while a CRC generation circuit 17 is activated when the selection signal 32 is logical '0' conversely with the CRC generation circuit 16 and both the circuits generate a CRC.