To retain an environmental atmosphere well and achieve energy saving by reducing the blow-out of inside air as well as the suction of outdoor air through the opening unit of a casing.

A fan F1 is arranged in the supplying duct 42 of drying air which is communicated with and connected to the casing 40, to supply the drying air into the space S of the casing 40 by operating the fan F1. A suction air adjusting damper 44 capable of adjusting the amount of drying air supplied into the space S is arranged in the supplying duct 42 at the upstream side of the arranging position of the fan F1. An outer air amount sensor 58 is arranged at the outside of the outlet port 40b of the casing 40. A detecting signal from the outlet air amount sensor 58 is inputted into a controller 46 while the controller 46 adjusts the opening degree of the suction air adjusting damper 44 based on the detecting signal.