PURPOSE: To transmit millimeter waves without disturbing the basic mode of oscillation and also obtain electrical insulation by electrically insulating a collector electrode portion and an output window from each other using a slit portion, and hermetically sealing the outer peripheral portion of the slit portion using a manifold connected to a vacuum exhauster.

CONSTITUTION: A plurality of micro spaces 10 are provided near the output window 7 of a collector electrode portion 4 and along the circumferential direction of a high frequency transmitter circuit in such a manner that the wall of the circuit located between a high frequency resonance circuit 3 and the output window 7 is partitioned in the cross direction of a tube axis (c). Each of the micro spaces 10 is formed by an insulating spacer 13 and metallic rings 15 and is surrounded by a manifold 11 hermetically coupled therewith and is connected to a super high vacuum exhauster via a vacuum exhaust pipe 12. High frequency leakage from each of the micro spaces 10 is thereby reduced as mush as possible and at the same time exhaust conductance sufficient to obtain super high vacuum can be obtained without disturbing the basic mode of oscillation and also the collector electrode portion 4 and the output window 7 are electrically insulated from each other.