To collect the heat of an inflow tube and an outflow tube heated by soil heat, by a heat medium with high efficiency.

This U-shaped tube 10 comprises the inflow tube 11 for allowing the heat medium F to flow from a ground surface L side toward an underground side, and the outflow tube 12 for allowing the heat medium F passing through the inflow tube 11 to flow from the underground side toward the ground surface L side, and the heat of the inflow tube 11 and the outflow tube 12 heated by soil heat is collected by the heat medium F by allowing the heat medium F to successively pass through the inflow tube 11 and the outflow tube 12. A guide member 13 is mounted to at least one of the inflow tube 11 and the outflow tube 12 to guide the heat medium F toward its inner peripheral face.