To provide a building air-conditioning system by geothermal use, for air-conditioning a building, while saving energy and reducing an environmental load, by effectively using underground heat.

A multitubular flat heat exchanger is formed by connecting an intake side header 21A and an exhaust side header 21B in an airtight state by a large number of metallic small diameter pipes 22, and is composed of at least one multitubular flat heat exchanger 20 buried in the ground of the required depth, an intake pipe 25 for introducing air-conditioning object air to the intake side header of the shell and tube flat heat exchanger, an exhaust ventilation pipe 26 having an air blowing mechanism 27 by connecting to the inside of a space for air-conditioning from an outlet of the exhaust side header of the flat heat exchanger 20, and an air-conditioning air blowoff part 8 arranged along a corner part of an indoor structural material via a proper space from the outlet side of the exhaust ventilation pipe 26.