PURPOSE: To decrease the transmission loss by setting the space of the helical winding forming the helical waveguide to the optimum value between the amount equivalent to one piece of the winding and the tight winding.

CONSTITUTION: The helical waveguide consists of helical winding 1, matching layer 2, masking layer 3 and reinforcement layer 4 each. In this case, space S of winding 1 is not set to the amount equivalent to one piece of the winding or the tight winding but to the intermediate value. In other words, space S is set within the range of O<S<D, where D is the outer diameter of winding 1. In such way, the sum amount can be minimized between the mode conversion loss caused by the bend of the tubular axis and that caused by the oval malformation of the cross-section. Thus the optimum design becomes possible to be applied suitably to the transmission line for the millimeter wave through the submillimeter wave.