Provided is a backward-wave oscillator in a communication system, including a waveguide formed of a metamaterial. A unit structure of the waveguide may include: a top plate; a short-circuited stub; a bottom plate separated at a predetermined gap from the top plate, and having the short-circuited stub formed in the center thereof; a first metal pillar connecting the top plate at a first port positioned on one surface based on the short-circuited stub to the bottom plate at a second port positioned on the opposite surface of the first port based on the short-circuited stub; and a second metal pillar separated from the first metal pillar, and connecting the top plate at the second port to the bottom plate at the first port.