To provide the shape capable of improving heat collecting and releasing effect and stability under the ground, with respect to a fin disposed on a tank crust portion buried under the ground and exchanging heat by utilizing geothermal heat.

This spiral fin disposed on the tank buried under the ground and the like to exchange heat of stored fluid by utilizing geothermal heat, is provided with an auxiliary blade, thus its surface area is increased, and the stability under the ground can be secured.