PURPOSE: To easily construct a closing means by providing gas compressing/ feeding means, a gas extracting means and gas extraction sections having the closing means in multiple storage spaces, and providing a gallery between the gas extraction sections and the ground surface.

CONSTITUTION: Storage spaces 7A, 7B are provided in the base rock, and gas extraction sections 7Aa, 7Ba are provided at a uniform interval in the spaces 7A, 7B. The closing means of a closing plug 20 is provided to close the extraction sections 7Aa, 7Ba and make the pressure receiving areas of the first pressure receiving face 21 and the second pressure receiving face 22 equal, and a communicating pipe 23 is provided on the plug 20. Compressed air 17 is fed to the storage spaces 7A, 7B with gas compressing/feeding means such as a compressor 11, a vent pipe 15, and a valve V1, and the compressed air 17 stored in the storage spaces 7A, 7B is extracted with gas extracting means such as the vent pipe 15 and a valve V2. The pressure receiving areas of the plug 20 are made equal at the same interval, and the plug 20 can be easily constructed.