PURPOSE: In a storage chamber equipped with refrigerating devices, a covering of airtight film is prepared in a form just like mosquito net to form an airtight space whereby the additional aging of banana can be done with a simple means.

CONSTITUTION: Inside the storage chamber 1 which has thermally insulating structure by use of insulating material as the walls 11, a refrigerating device is provided and a covering of airtight film such as vinyl film is prepared in a form like Japanese style mosquito net to form an airtight space A by immersing the bottom ends 32 of the covering 3 in water of ditches 33. A controller for the temperature and gas composition and a gas circulator are set in the space A and green-aged banana is aged in the airthight space A. The use of the space A permits the gas composition to change to more suitable one in the course of aging at desired periods and further enables the appropriate temperature control which is rising as the aging progresses.