To provide an easily constructible system and a separator suiting the system in regard to a geothermal heat utilization system vertically fractionating an interior of a well casing buried underground into a water pumping area and water returning area by the separator, and carrying out pumping and returning of underground water by a single well.

Since each piece of piping 6-9 suspended by a casing lid 5 is fixed to a plurality of bracing guides 26 disposed at predetermined intervals in a vertical direction, the pieces of piping 6-9 and the separator 2 can be treated as one shaft. By this, when constructing the geothermal heat utilization system, the pieces of piping 6-9 and the separator 2 can be easily inserted into the well casing 1 buried underground. Since three collar parts 15 formed so as to overlap in the vertical direction are provided on a packing part 14 of the separator 2, even if distortion is generated in the well casing 1, each collar part 15 adheres to an inner face of the well casing 1, and water-tightness between the well casing 1 and the separator 2 can be secured.