A control unit is added to the environmental tester including a thermostat-humidistat container, an air conditioner, a first temperature and humidity sensor for measuring temperature and humidity in the thermostat-humidistat container, a controller for receiving a first signal from the first temperature and humidity sensor to control the air conditioner. The control unit includes a second temperature and humidity sensor configured to measure the temperature and humidity distribution in the thermostat-humidistat container, and an additional unit configured to receive the first signal and set values for the temperature and the humidity from the controller, and receive a second signal from the second temperature and humidity sensor, compute a correction value from the received first signal, second signal, and set values, and send the correction value to the controller.