Generally, the dew condensation generated in the tunnel does not directly affect the concrete structure However, falling water due to dew condensation brings directly with a high possibility of defection to the train line. In addition, moisture inflow due to dew condensation may induce malfunction or failure of various electric and fire facilities installed in the tunnel, which may cause abnormal signals or accidents and so on. In this paper, we explain the case of dew condensation site and analyzed the cause.