When a microstrip line is connected with a waveguide, there is a limit to reducing the connection loss by using only a matching box. We have discovered that in a transmission mode line transducer for converting between the TEM waves of the microstrip line and the TE01 waves of the waveguide, if the cross-sections of the microstrip line and the waveguide are substantially the same size, in the case of a 50Ω microstrip line when the characteristic impedance of the waveguide is about 80%, i.e., 40Ω, the line conversion loss can be optimized. Therefore, according to the present invention, the microstrip line is connected with the waveguide using a λ/4 matching box by means of a ridged waveguide having a low impedance and a length of λ/16 or less.