Abstract: The transmission device of the present embodiment includes a waveguide unit which transmits a terahertz-wave signal, and a plurality of ports provided around the waveguide unit and each composed of a waveguide and a planar lens, the waveguide unit and the ports being integrated on a planar substrate with dielectric properties. The planar lens diffuses, in an arcuate shape, a terahertz-wave signal by a reflective index set by a staggering arrangement of first through-holes, transmits the diffused terahertz-wave signal to the waveguide unit in parallel, or focuses a terahertz-wave signal which is transmitted in parallel through the waveguide unit. A beam splitter transmits a terahertz-wave signal, which is transmitted in parallel from a first planar lens, to a second planar lens, by reflection or transmission by a refractive index set by a grid arrangement of second through-holes.