PURPOSE: To stabilize the cooling performance, prevent occurrence of a discharge, and prevent breakage of an RF cavity by bringing one end of a heat pipe into contact with a heat generating part of an RF cavity, and exposing the other end part outside of the RF cavity.

CONSTITUTION: A heat pipe 11 in contact with an inner wall of a coaxial tube 3 at one end thereof and exposed protruding out of the tube 3 at the other end part is disposed to extend vertically. A power coupler 12 comprising the tube 3, a high frequency window 4, a coupler antenna 6, and the pipe 11 becomes a heat generating part of an RF cavity. Generated heat is absorbed at one end part, that is a lower end part, of the heat pipe 11 and transmitted upward to be radiated from an upper end part. The power coupler 12 is thus cooled by the pipe 11. Occurrence of a discharge by deterioration of the cooling performance can thus be prevented, and breakage of the RF cavity is also prevented.