A millimeter wave transmission device, the millimeter wave transmission device having (a) a first signal processing board for processing a millimeter wave signal; (b) 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 (c) 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 member constitutes a dielectric transmission path via which the millimeter wave signal is transmitted between the first signal processing board and the signal processing board.