To obtain a transmission line of high frequency with small transmission loss, and to relax the machining precision of a conversion part and to improve the assembly by providing a choke structure between a microstrip line and a coaxial line.

To hole a dielectric substrate 2, a chassis 4 is provided on its surface facing a ground conductor 1, and the chassis 4 and ground conductor 1 are adhered and fixed with a conductive adhesive 5. A center conductor 6 is provided at the end of the microstrip line 3 perpendicularly to the circuit surface of the microstrip line 3 and partially projects from the chassis 4. The coaxial line 9 consists of a cylindrical dielectric 7 provided at the outer periphery of the center conductor 6 and an external conductor 8 provided at the outer periphery of the cylindrical dielectric 7. Further, a choke part 11 is provided at the outer peripheral part of the external conductor 8 constituting the coaxial line 9. A radio wave transmitted by the microstrip line 3 is fed to the center conductor 6 of the coaxial line 9 and passed through the coaxial line 9, and then transmitted to other circuits by connecting the part partially projecting from the chassis 4 to other-circuit's components.