A redundant control device of a high voltage direct current (HVDC) transmission system is provided. The redundant control device includes a first system and a second system that performs a function corresponding to that of the first system; and a first change over logic (COL) and a second COL that determine whether to switch between the first system and the system, wherein there are optical communication modules between the first system and the first COL, between the first COL and the second COL, and between the second system and the second COL to transmit and receive data, and the first COL and the second COL includes a monitoring control unit that checks a connection or wiring state of the optical communication modules and validity of transmission data.