To provide an inexpensive dehumidifying apparatus and a detection program of anomaly in drain discharge correctly detecting the anomaly in drain discharge.

The compressed air dehumidifying apparatus 1 runs compressed air around an evaporator 17 of a refrigeration cycle 18 having a compressor 11 compressing a coolant and the evaporator 17 cooled by evaporation of the coolant, and cools the compressed air and condenses moisture in the compressed air to discharge as drain. The apparatus 1 is provided with a drain channel temperature sensor 26 detecting temperature disposed in a drain discharge channel in the downstream of a drain discharging solenoid valve 24 discharging drain; and a control part 4 controlling opening and closing of the drain discharging solenoid valve 24, and determining drain discharge anomaly based on temperature difference of respective temperatures detected by the drain channel temperature sensor 26, when the drain discharging solenoid valve 24 is controlled to a close state from an open state and after elapse of a first given time from the close control, while the refrigeration cycle 18 is operated in given conditions.