regulator 46 is set correspondingly to a signal from the oxygen concentration setting control unit 22 and the number of revolutions per minute of the blower 14 is controlled in accordance with the frequency since the construction and operation of the other parts of this embodiment are the same as those of the corresponding parts of the first embodiment the descriptions thereof are omitted fig7 and 8 show a third embodiment of the present invention which is obtained by combining the first embodiment of fig1 with the second embodiment of fig6 in the third embodiment a change over operation is carried out by operating a circuit change over control valve 51 when the oxygen concentration is regulated by positively supplying oxygen o 2 or nitrogen n 2 to the interior of the portion of an outer air circulating passage 4 which is on the upstream side of a blower 14 in accordance with a signal outputted from an oxygen concentration setting control unit 22 correspondingly to the oxygen concentration in a test oven body 1 or when the oxygen concentration is regulated by controlling an air replacement rate with the atmospheric air introduced from an atmospheric air supply passage 50 connected to the portion of the outer air circulating passage 4 which is on the upstream side of the blower 14.