To provide a highly efficient heat exchanger with no risk of the occurrence of a pitch deviation in a pipe of the heat exchanger, t facilitating working and construction even when buried.

An inner pipe 3 provided with a spiral groove 9 on the outer face is inserted in a cylindrical outer pipe 5. The outer diameter (outer diameter of a projection part 11) of the inner pipe 3 and the inner diameter of the outer pipe 5 are substantially the same. Therefore, the projection part 11 of the inner pipe 3 and the inner face of the outer pipe 5 come in contact, and a spiral flow passage surrounded by the groove 9 and the inner face of the outer pipe 5 is formed. When a fluid is made to flow in the flow passage, the fluid flows in the spiral flow passage formed by the groove 9 and the inner face of the outer pipe 5, and performs heat exchange with the surrounding ground heat.