Abstract: A coaxial line-to-microstrip line transition device including a coaxial line, a microstrip line circuit, and a conductive connection part. The inner conductor of the coaxial line is connected with a microstrip line of the microstrip line circuit. The connection part includes a semi-cylindrical plate which is connected with the outer conductor of the coaxial line by grasping it, and a flat plate which is connected at a surface thereof with a earth layer surface of the microstrip line circuit. The microstrip line is mounted on one surface of a dielectric substrate, and the earth layer is mounted on the other surface of the dielectric substrate. The dielectric substrate is mounted on a chassis which is either provided separately or shared by the flat plate of the connection part which is extended in size. Preferably, a through-hole is provided at least one of the semi-cylindrical plate and the flat plate to be filled with a conductive material.