This paper reports the development of a multivariable control system for the energy saving of a thermo-hygrostat. The thermo-hygrostat is used to inspect various products and to create the desired atmosphere by controlling the temperature and humidity in the chamber. The controlled values are those of temperature and humidity, and manipulated values are those of the heater, humidifier and refrigerator. A conventional multi-loop PID controller is known to have sometimes caused energy loss owing to the canceling out of the manipulated values because the system gains of both the heater and the refrigerator are opposite. In this study, the proposed control system was able to achieve a reduction in the energy consumption by also controlling the output of the refrigerator; the experimental results show this decrease.