There is a possibility that the mixture gas of air and helium diffuse into the reactor core at the depressurization accident of a HTGR. In order to investigate a method to control natural circulation flow of air and helium, we have conducted an experiment using an apparatus that simulates the flow path configuration of the HTGR. The experimental apparatus consists of two vertical channels with rectangular cross-sections, a connecting channel at the top of the channels, and a storage vessel at the bottom of the channels. The temperature of the heated and cooled walls and the temperature of the mixture gas in the vertical slots were measured. The concentration of air was also measured by ultrasonic concentration meter. From the results obtained in this experiment, it was confirmed that the natural circulation flow stopped for a while after helium gas injection. We will also describe the results obtained in the three-dimensional numerical analysis.