To provide an environment control method and a thermo-hygrostat capable of stably adjusting an air temperature and humidity to a low temperature-high humidity state with high accuracy.

In the environment control method for conditioning the air temperature to a final target temperature lower than an air temperature in an initial state, and adjusting the air humidity to target humidity, air absolute humidity is adjusted to target absolute humidity first, and then, the air temperature is adjusted to the final target temperature. The thermo-hygrostat 1 has a shape formed by connecting two thermo-hygrostats A, B by ducts 6, 8. The hygrostat A is a device creating the air of the target absolute humidity, incorporating an intermediate environment coordinating section 2, and having an air chamber 10 and a first air adjusting means 11. The hygrostat B is a device creating the final target environment, having a space to place a test artifact, and composed of a final environment coordinating section 3 and a test room 5.