PURPOSE: To largely reduce the transfer of the heat generated by a collector section to a focusing magnetic field device section through a case substrate by providing a heat nonconductor between the focusing magnetic field device section of the case substrate and the portion where the collector section is fitted.

CONSTITUTION: An electron gun section 4 generating an electron beam, a focusing magnetic field device section 6 generating the magnetic field to focus the electron beam, and a collector section 7 capturing the electron beam passing a high-frequency circuit section 5 and converting it into thermal energy are fitted on a case substrate 1. A case 3 is fitted to the case substrate 1 to cover the whole, the case substrate 1 is made of a material with high thermal conductivity such as Al, and a heat nonconductor 2 is made of alumina ceramic and the like. When a linear beam microwave tube is operated, the heat generated by the collector section 7 is diffused to the outside through the case substrate 1, however the heat quantity transferred to the focusing magnetic field device section 6 can largely be decreased because the heat nonconductor 2 is provided.