Abstract: Provided is an in-millimeter wave dielectric transmission device including a first signal processing board for processing a millimeter wave signal, a second signal processing board signal-coupled to the first signal processing board to receive the millimeter wave signal and perform signal processing with respect to the millimeter wave signal, and a viscoelastic member provided between the first signal processing board and the second signal processing board and having a predetermined relative dielectric constant and a predetermined dielectric dissipation factor. The viscoelastic member constitutes a dielectric transmission path. With such a configuration, the viscoelastic member absorbs vibration when external force is pplied to the signal processing boards, so that! vibration of the first signal processing board and the second signal processing board can be reduced, and a millimeter wave signal between the signal processing boards can be transmitted through the viscoelastic member at a high speed without using connectors and cables.