PURPOSE: To always keep the outlet temperature of a heat exchanger constant, by providing a turbulence promotion body, being moved by an energizing means, made of shape memory alloy, between the inlet and the outlet of a heat exchanger.

CONSTITUTION: The temperature at the outlet 15 of a heat exchanger is increased when the temperature in exhaust gas at the inlet 14 is increased. When the temperature at the outlet 15 is increased, the primary energizing means 18 made of shape memory alloy is returned to the memorized state which is an extended state. A turbulence promotion body 17 is moved to the right side and also to the axial direction by the intermediary of a movable plate 19, accompanied by the primary energizing means 18 when it returns to the memorized shape. As a result, the efficiency of a heat exchanger, that is the thermal efficiency, is increased, so that the temperature at the outlet 15 is controlled with the specified range which is set as a target at the beginning.