The purpose of this study is to clarify the thermal performance of an earth tube and its possibility as a heat source during the winter and a heat sink during the summer for an air-to-air heat pump system. The experiment was done in summers and winters over a period of two years. This paper principally reports the results obtained in the first winter. An earth tube with a diameter of 20 cm and a length of about 40 meters is embedded in the earth 1.0 meter below the ground surface around a test house to be heated. That tube is connected with a chamber, where the outside unit of the heat pump is installed. Measurements show the two-year earth temperature changes around the earth tube and the COP of the heat pump during the winter. The fact that the outdoor temperature at which the COP of the heat pump with the earth tube is more than that when the tube is not used is discussed.