A model to analyze tracer test data is proposed. It consists of a fractured layer and a slightly-fractured layer, where tracers move between the layers in proportion to their concentration gradients. It is shown that the slightly-fractured layer plays an important role in the movement of fluids and transport of tracers in fractured rock masses. The concepts behind the proposed model for identifying geothermal reservoir structure from the analysis of tracer returns and its application to field data are described. The results demonstrate that one main flow path can explain tracer responses showing two peaks or a long tail. The model allows us to calculate the flow fraction and the fluid flow velocity through the slightly-fractured layer from the tracer returns observed in geothermal wells.