PURPOSE: To eliminate the possibility of forming the stagnation part of a coolant in a coolant refluxing passage and smoothly send the coolant by refluxing the coolant in a refluxing passage between an outer tube and an inner tube and in the internal passage of the inner tube at the time of heating treatment by microwaves.

CONSTITUTION: A coolant refluxing means for continuing a ring passage 14 between an outer tube and an inner tube to the internal passage 15 of the inner tube, and refluxing a coolant in the ring passage 14 and in the internal passage 15, is provided. Namely, at the time of heating treatment by microwaves, the coolant is refluxed in the ring passage 14 between the outer tube and the inner tube and in the internal passage 15 of the inner tube. Thus, the stagnating part of the coolant is never formed in the coolant refluxing passage, the coolant can be smoothly sent, and a part of the tube cavity can be prevented from being unnecessarily locally heated.