A train car-to-car communication device includes: a terminal; an intra-composition transmission line connected to the terminal within the same composition for transmitting information; a composition-to-composition communication relay unit connected to the intra-composition transmission line for receiving first information from the terminal within the composition and transmitting the first information to an adjacent composition, and receiving second information from the adjacent composition and transmitting the second information to the terminal within the composition of interest; and a composition-to-composition transmission line for connecting the composition-to-composition communication relay units of the adjacent compositions and transmitting the information, and the composition-to-composition communication relay unit determines a difference between composition directions of the composition as a transmission source of the received information and the composition to which the composition-to-composition communication relay unit of interest belongs, and rewrites to invert direction-dependent information contained in the received information if the composition directions are different.