To easily install a hollow pipe (space) below an underground water level and to facilitate the management of internal cleaning, deodorizing, mildewproofing, etc. in a geothermal utilization air-conditioning system.

In installing a hollow pipe (space) in the ground below the underground water level for the purpose of the geothermal utilization air-conditioning system, after water injection, insertion and fixation in the pipe (space), high pressure foam (mixture of high pressure air and a surfactant solution or the like) is injected into an inner bottom to eliminate water in the pipe. High pressure foam (the mixture of the high pressure air and surfactant solution or the like) mixed with a detergent, an antimicrobial agent, lactic acid bacteria, etc. is injected into the hollow pipe (space) in the ground to contribute to cleaning, deodorizing and mildewproofing. When mixing lactic acid bacteria beforehand in a functional agent such as the detergent, the antimicrobial agent is also mixed to prevent the deterioration of the functional agent caused by the lactic acid bacterial.