PURPOSE: To provide the digital multiplex converter by which a fault location is momentarily confirmed.

CONSTITUTION: A data input side of a multiplex data transmission means provided with a multiplexer section 8 is provided with plural unipolar conversion means 20 converting bipolar signals of plural systems into unipolar signals. The unipolar conversion means 20 consists of a distribution section 1 receiving a one-system of bipolar signal and distributing it into two systems, two unipolar conversion sections 2, 3 converting the bipolar signal into unipolar signals, two alarm detection sections 4, 5 establishing synchronization of input data and detecting an alarm such as signal input interruption, out of synchronism or occurrence of an error with synchronization data information in the input data from a low order group based on outputs of the unipolar conversion sections 2, 3, a selector section 7 selecting either of outputs of the two unipolar conversion sections 2, 3 and a selector control section 6 selecting a normal system based on the two systems of alarm detection sections 4,5 and commanding the selection of the channel to the selector section 7.