PURPOSE: To reduce the manufacture man-hours and to decrease the production cost while keeping the electric performance by forming an inner wall corner of a hollow part to be a circular-arc shape.

CONSTITUTION: Rectangular waveguides 2, 3 have respectively rectangular hollow parts 2a, 3a whose inner wall corner is formed to be circular-arc. Then the rectangular waveguide 2 is arranged so that its rectangular hollow part 2a is tilted by 26.4° with respect to the rectangular waveguide 1 and the rectangular waveguide 3 is arranged so that its rectangular hollow part 3a is tilted by 63.6° with respect to the rectangular waveguide 1. Thus, the rectangular follower parts 2a, 3a are formed by cutting by a milling machine or the like by forming the inner wall corner of the rectangular hollows 2a, 3a to be a proper circular-arc. That is, the discharge processing process is omitted in the manufacture of the rectangular waveguides 2, 3 and the manufacture man- hours are reduced. Furthermore, since the rectangular waveguides 2, 3 are manufactured by the cutting only without use of the electric discharge machining, the production cost is reduced.