PURPOSE: To prevent deterioration of an insulating oil heated by an electron gun of a gyrotron oscillating tube so as to enable long time operation by providing a cooling plate able to let a cooling medium flow to a socket.

CONSTITUTION: A cooling plate 10 is mounted on a socket 5 so as to concentrically enclose an electron gun 2 and a cooling pipe 11 is circumferentially installed on the cooling plate 10 while an insulating oil as a cooling material is delivered from an oil circulating pump 9 through a cooling piping 12. Thereby, the insulating oil heated by the electron gun 2 is cooled by the cooling plate 10 in which the low temperature insulating oil is running so as to be able to prevent deterioration of the insulating oil due to local heating and the damage of a cryostat while enabling long time operation.