To provide a heat transfer tube for underground heat exchange capable of reducing equipment costs and pump power costs by improving heat exchanging efficiency and shortening a length of the heat transfer tube, reducing a time and costs for excavating by shortening and reducing a size of a hole for burying, and easily inserting the heat transfer tube into the hole as well as back filling.

This heat transfer tube for underground heat exchange comprises a pair of resin flat heat medium tubes 1, 1 of a prescribed length mounted in opposition to each other, and a joint portion 2 for communicating and connecting the pair of flat heat medium tubes 1, 1 so that a heat medium flows while U-turning. A major axis side of the flat heat medium tube 1 is formed into the sharp shape, and the pair of flat heat medium tubes 1, 1 are mounted in such a state that their major axis sides are intersected with each other.