Provided is an electromagnetic wave transmission medium which is suited for mass production and does not affect a transmission mode. The electromagnetic wave transmission medium includes, as a main element, a flexible cylindrical tube (1) molded so that a cross-sectional shape of the cylindrical tube in a direction orthogonal to a tube axis is uniform in a direction of the tube axis. The cylindrical tube (1) includes an inner wall formed of a conductive layer having a thickness equal to or more than a skin depth. The cross-sectional shape is a circular ridge waveguide shape having a ridge (1b) which is oriented to a cylindrical axis and is symmetric with respect to a center, and the ridge (1b) has a structure to be fed with electricity.