A serial communication device has: a MAC (Media Access Control), a mask circuit and a buffer. The MAC conforms to Serial Media Independent Interface specification and outputs an identical segment data for plural times within a unit period. The mask circuit is configured to mask a predetermined segment data out of the plurality of identical segment data and to output the other segment data out of the plurality of identical segment data. The buffer is configured to receive the segment data output from the mask circuit and to output the received segment data to a physical layer device in synchronization with a clock signal.