PURPOSE: To relatively easily control the humidified state in a chamber by introducing mist generated by an ultrasonic humidifier into a chamber to effect forced humidification in the chamber and to surely prevent the drying of the preserved dough before fermentation by automatically controlling the introduction of mist into the chamber.

CONSTITUTION: A refrigerator 1 for holding dough before fermentation is provided with an ultrasonic humidifier 20. The mist generated by the ultrasonic humidifier 20 is introduced into the chamber 3 through ducts 21a,21b. A time- limit contact of a timer for setting the working period and the pause period is placed in the driving power circuit of the ultrasonic humidifier 20. The mist generated by the ultrasonic humidifier is introduced into the chamber to effect forced humidification of the chamber and enable sure prevention of the drying of the preserved dough to be fermented and the relatively easy control of the humidified state in the chamber.