PURPOSE: To simplify the device and to improve the reliability by providing a notch at a circular waveguide to guide an electromagnetic wave, reflecting the electromagnetic wave radiated from the notch on an annular mirror, and making the electromagnetic wave incident on the notch of the circular waveguide in a mode converter.

CONSTITUTION: A concave mirror 38 is divided into the same pieces as the number of circumferential direction modes 12 of an input electromagnetic wave with shape periodically changing in a circumferential direction. A step 40 is provided at the boundary of each divided reflecting surface 39. Then, a unit normal vector set on the divided reflecting surface 39 is formed so as to satisfy a specific condition. Meanwhile, the appropriate position of the notch 37, the diameter, and the tapered angle of a tapered circular waveguide 36 are set so that the distribution of the electromagnetic field of the electromagnetic wave reflected on the annular mirror 35 can approach that of a TE01 mode at the notch 37. As a result, it is possible to transmit the electromagnetic wave with whispering gallery mode by directly converting to the TE01 mode with the mode converter 32.