PURPOSE: To enable discharging temperatures of fluid of high temperature side and of fluid of low temperature side passing through a heat exchanger to be varied within a wide range by a method wherein a partition plate is disposed at a thermal insulation part of a closed thermal siphon, an inner down-pipe is disposed at the partition plate toward a heating part, an inner up-pipe is disposed at the partition plate toward a cooling part and a valve capable of varying a flow rate of working fluid flowing in the inner down-pipe or the inner up-pipe is provided.

CONSTITUTION: In a closed type two-phase thermal siphon, a height (T) of an inner up-pipe 5 keeps such a height as one in which liquid in a liquid accomulator A is not back-flowed and this is determined in response to a charging amount of the initial working fluid. A degree of opening C of a valve 7 is adjusted and an inner circulation amount of the working fluid is adjusted, thereby a rate of a boiling region I and a dry-out region H generated in the heating part 3 can be varied, resulting in that a thermal conducting amount through the thermal siphon can be varied approximately to zero.