Disclosed are an optical module interfacing device for connecting an RJ interface and an SFP type optical module to an SFP type optical module connector without additional processing, and an Ethernet system using the optical module interfacing device, thus supporting data transmission and reception. The optical module interfacing device includes a board having the same dimensions as those of the optical module, a male connector, having the same dimensions as those of the optical module, formed at a side surface of one end of the board to be connected to the female connector for the optical module mounted on a host board, and an RJ female connector provided with a plurality of pins formed on an upper surface of the board, respectively corresponding to pins of the male connector, according to standards. Further, UTP data is provided to MDI supporting ports of a physical layer through an interfacing unit.