PURPOSE: To secure the same phase velocity for both the odd and even mode waves by drilling the hole at the area corresponding to the strip line and then filling the material having the higher dielectric constant than that of the dielectric into the hole, thus improving the directivity.

CONSTITUTION: Dielectric 2 is put onto earthed conductor 1, and microstrip lines 4 and 5 plus dielectric 3 are deposited onto dielectric 2. These lines 4 and 5 are distributed in parallel each other to conductor 1 in the form of the parallel planes. Then the hole is drilled to dielectric 2 corresponding to lines 4 and 5 and against dielectric 3, and then the material having the higher dielectric constant than dielectric 2 is filled into the hole. Thus a directional coupler is formed. The electromagnetic input is applied to input terminal IN of the coupler to produce the output signals from output terminals 11 and 12. And then the logic odd and even mode electromagnetic waves are propagated in the same velocity and along lines 4 and 5 in order to secure the same phase along the parallel field coupling regions. Thus the improvement is secured for the directivity.