m 3 as seen the frequency that the absolute humidity in the surrounding atmosphere becomes in excess of the set value of absolute humidity is low but it is considered that the absolute humidity in the surrounding atmosphere in excess of the set value of absolute humidity is observed when the drying operation is started for the first time in the morning or during the rainy season especially in linen supply factories there is a tendency toward complete automation from washing process to drying process without any attending operator so that the whole operation is very adversely affected if there are no means for preventing the absolute humidity in the surrounding atmosphere from becoming in excess of the set value of absolute humidity even if the frequency of such phenomenon is low based on the time characteristic shown in fig1 when the absolute humidity in the surrounding atmosphere is higher than the set value of absolute humidity according to the present invention the decrease in the drop rate of a degree of absolute humidity detected by the absolute humidity sensor 51 is detected so that the absolute humidity in the surrounding atmosphere is determined higher than the set value of absolute humidity thereby correcting the set value of absolute humidity in excess of the absolute humidity in the surrounding.