This invention relates to vehicle mounted CAN bus fiber concentrator and to a serial communication device between car inner devices. It comprises N pieces of plastic sending and receiving and lossless arbitration circuit 1 and signal exchanging circuit II. The light signals sent from the plastic fiber is converted into electric signals through plastic fiber receiving circuit and computes through the photoelectricity receiving circuit related line and logic diode and then are converted into electric signals suitable to the plastic fiber sending circuit through signal conversion circuit. Finally the electric signals are converted into light signals through fiber sending circuit to realize the CAN bus plastic fiber transmission.