Vehicle-mounted relay connection unit

A relay connection unit, to be mounted on a car, which relays messages received from electronic control units connected to a plurality of first CAN communication lines respectively to electronic control units connected to a second CAN communication line having a higher transmission speed than said first CAN communication lines in a cut-through system,

• said relay connection unit comprising a sending/receiving part connected to said first CAN communication lines and said second CAN communication line;
• and
• a relay processing part connected to said sending/receiving part,
• wherein regarding messages said relay processing part receives simultaneously from a plurality of said first CAN communication lines, from a time difference between a finish time point when reception of a control field of one of said messages has finished earliest and each of finish time points when receptions of control fields of other messages have finished and a transmission wait time period from said finish time point when said reception of said control field of each of said messages has finished, said relay processing part finds a sum of said transmission wait time period and said time difference at said reception finish time points for each message and first relays one of said messages having the smallest sum of said transmission wait time period and said time difference to said second CAN communication line.