The present invention includes a dehydration method comprising the steps of providing a distilled process-target fluid to a water separation membrane device via a heat exchanger; separating the process-target fluid into a dehydrated product and water by using the water separation membrane device; detecting a temperature of any one of the water separation membrane device and the process-target fluid supplied to the water separation membrane device by using a temperature monitoring device; and controlling the temperature of the process-target fluid so that the temperature of the distilled process-target fluid being maintained at a temperature higher than the condensation temperature of the distillate by 5 to 10° C. by using a temperature adjustment device provided in the water separation membrane device.
