PURPOSE: To construct a transmission system provided with redundancy at a low cost by repeating and controlling only one system in a proper system transmission line to the terminal equipment side by a selective control part in a transmission switching device.

CONSTITUTION: The switching device-is provided with two transmission diagnostic parts UA, UB, the selective control part U1, four relays RA, RA, RB, RB, input terminals UT1, UT2 for a 1st system transmission line LA, input terminals UT3, UT4 for a 2nd system transmission line LB, output terminals UT5, UT6 for the line LA, and output terminals UT7, UT8 for the line LB. Whether respective data signals from the plural system transmission lines LA, LB for obtaining redundancy are proper or not are respectively diagnosed by the diagnostic parts UA, UB. Then only one system in the proper system transmission line is repeated and connected to the terminal equipment side by the control part U1. Even when each terminal equipment is not provided with a redundant processing mechanism, redundancy can be obtained as the whole system.