PURPOSE: To decrease leakage of millimeter wave, by providing the plural number of micro gaps on the wall of a transmission circuit located between a high frequency resonance circuit and an output window, in a direction of crossing a tube axis, and providing a manifold which is coupled to a vacuum exhausting device on outside periphery of the micro gaps.

CONSTITUTION: A millimeter wave device such as a gyrotron is composed of an electron gun part 1, a beam tunnel part 2, a high frequency resonance circuit part 3, collector electrode part 4, an output window 7 and a vacuum exhausting device 12. In this case, the plural number of micro gaps 10 are provided on the wall of a high frequency transmission circuit located between a high frequency resonance circuit 3 and an output window 7 in a direction as crossing a tube axis, and their outside periphery is airtightly sealed by a manifold 11 which is coupled to a vacuum exhausring device 12. And conversion of a TEon mode which is the generation basic mode to an unnecessary mode is suppressed and sufficient exhausting conductance is formed. Thus, the minimum high frequency leakage can be achieved even when a high power is generated.