PURPOSE: To hold sufficient electric power capacity of a load for the second cavity and further form a coil in a small size, by mounting a dielectric member to a part of the upper or bottom wall in at least one or more cavities of a cavity resonator, thus enabling any leakage of electromagnetic wave to be absorbed.

CONSTITUTION: A circular or oval shaped hole is formed on a part of the upper wall 16 of the second cavity, and a dielectric member (for instance, ceramic) 17 is mounted in such a manner as to block said hole. This dielectric member performs action such that a klystron is held in a vacuum state while an electromagnetic wave in the second cavity is partly leaked in a space 12 under circulation of water and a factor Q of said cavity is decreased. Because of the construction in which water is directly brought in contact with the upper face of ceramic, it is easily understood that very large electric power capacity is provided.