Described herein is a desiccant air-conditioning system which is characterized in that the system is equipped with an introducing route where process air is introduced into a geothermal heat exchanger, a supplying route where the process air which is cooled by the geothermal heat exchanger is introduced into a space to be air-conditioned and a desiccant wheel which is placed in an extending mannerover both of the introducing route and the supplying route and, when the desiccant wheel is rotated so that each part thereof is successively positioned to said introducing route and supplying route,the process air after cooling is dehumidified by said desiccant wheel and the desiccant wheel is regenerated by the process air before cooling.