A dielectric waveguide designed to avoid the influence of reflection of electromagnetic waves at connected portions of dielectric strips and to have an improved characteristic. A third dielectric strip is inserted in a part of a connection section at which a first dielectric strip and a second dielectric strip are connected to each other, and the distances between the three connection planes in said connection section are determined so that a wave reflected at the connection plane between the first and third dielectric strips, a wave reflected at the connection plane between the first and second dielectric strips, and a wave reflected at the connection plane between the second and third dielectric strips are superposed with a phase difference of 2&pgr;/3 from each other. Alternatively or in addition, the distance between the first-second dielectric strip connection plane and the first-third dielectric strip connection plane is set to ⅙ of the guide wavelength of an electromagnetic wave propagating through the dielectric strips, and the distance between the first-second dielectric strip connection plane and the second-third dielectric strip connection plane is set to ⅙ of the guide wavelength. Reflected waves are thereby superposed in phase opposition to each other to cancel out. In this manner, propagation of a reflected signal to ports is limited.