To provide a triple U-shaped tube type underground heat exchanger having enough strength so that it is not damaged by ground pressure even when it is buried about 300m under the ground rich in geothermal heat.

All of a first U-shaped tube 1, a second U-shaped tube 2 and a third U-shaped tube 3 for circulating antifreeze liquid, and a straight guide tube 4 have equal outer diameters, the first U-shaped tube 1 and the third U-shaped tube 3 are placed in such manner that a going way D and a returning way U are formed without clearance therebetween, and they are opposite to each other with the guide tube 4 in between. The second U-shaped tube 2 is placed in such a manner that a clearance portion G in which the guide tube 4 can intrude without clearance, is formed between the going way D and the returning way U, the guide tube 4 intrudes into the clearance portion G, and the going way D and the returning way U are respectively formed between the first U-shaped tube 1 and the third U-shaped tube 3. Adjacent parts of the first U-shaped tube 1, the second U-shaped tube 2, the third U-shaped tube 3 and the guide tube 4 are respectively brought into contact with or close to each other.