PURPOSE: To obtain a converter featuring less dispersion of the characteristics by securing such structure that has no relation with the insertion depth of the conversion line or the micro-strip line.

CONSTITUTION: The coaxial line composed of dielectric 11 and central conductor 10 is made to pierce through the H-face at the position which is distant away by 1/4 wavelength from the inner surface of the waveguide of short-circuit termination 2 of square waveguide 1. At the same time, the tip of the coaxial lines each is made to protrude over the outer surface of termination 2 through the tube wall of the H- face of the waveguide. Two tips of conductor 10 are connected to the end of micro- strip lines 12 and 13 each which have the difference of length by the amount equivalent to 1/2 wavelength, and then connected to the other end of lines 12 and 13 each. Then these joints are connected to another micro-strip line 14. The signal put into waveguide 1 features the maximum electromagnetic field distribution at the spot which returns from the short-circuit surface by the amount equivalent to 1/4 wavelength. And the signal is excited between conductor 10 and wall 3 of the waveguide to be transmitted in the coaxial mode.