Abstract: In conducting an environmental test of a vehicle, the vehicle is housed in the testing chamber and in which the vehicle is subjected to test simulating external environmental various conditions. In such test, it is often required to keep the vehicle stationary while varying a temperature and a humidity of the testing chamber. In such a case, the testing chamber provided with a by-pass air duct at a position near a ceiling portion of the testing chamber enables the vehicle being tested to be prevented from being subjected to an air flow caused by air-conditioning operation of the testing chamber, which improves a test in accuracy in case that the test must be conducted at a desired temperature/humidity condition.