PURPOSE: To make an thermal element to act quickly by reflecting the action of the thermal element to a draft temperature in a wind tunnel, even when air quantity and wind velocity fluctuate, and by preventing the.delay of the action of the thermal element, especially when air quantity has decreased.

CONSTITUTION: A thermal element 9 for preventing temperature from rising too much is placed on the discharge opening 4 side of a heater 8. A heat retaining plate 10, which opens the air passage 14 of the discharge opening side of the thermal element 9 when the air quantity of hot air passing by the neighborhood of the thermal element 9 increases, and which closes the air passage 14 of the discharge opening side of the thermal element 9 when the air quantity decreases, is mounted near the discharge opening 4 apart from the position of thermal element 9. When air is normally blowing, the air passage 14 of the discharge opening side of the thermal element 9 is opened, hot air directly blows to the thermal element 9, and consequently, the thermal element can act quickly if the temperature has risen too much. When the air quantity has decreased, the air passage 14 of the discharge opening side of the thermal element 9 is closed, heat retaining place is formed near the thermal element 9, and consequently, the temperature of the thermal element 9 rise enough to act quickly.