PURPOSE: To reduce the labor in a manufacturing process along with the reduction of dangerousness of vacuum leakage by vacuum-sealing a plural number of cavity resonators of a multicavity klystron with a single plate shaped parts.

CONSTITUTION: The vacuum-sealing of a cavity resonator is performed using a vacuum-sealing plate 6 obtained by previously bonding a plural number of bellows 4 and a plural number of movable side plates 3 by blazing them, and by brazing or welding a space between a cavity wall 2 and the vacuum sealing plate 6. Moreover, if one sealing plate 6 is then used, only surroundings of the sealing plate 6 are good to be bonded without bonding wall portions among resonators. In a klystron having four cavity resonators, its vacuum-sealing can be performed even with about 2/3 of the area of a conventional one, enabling the possiblity of vacuum leakage to be also reduced by the amount of the reduced sealing area.