To release a door chain in response to requirements even by anyone as men having a duplicate key from the outside in the door chain capable of being operated only by the men on the inside, and to enable the treatment of men including the aged in a house.

One or a number of parts nearest to a door in the chain are removed while the effect of the door chain is left as it is, and a lock such as a small-sized pad lock is used in place of one or a number of the parts. When opening the door, the man having the duplicate key inserts its hand from the space between the door and a building frame, and the door is opened using the duplicate key at a visual angle and by a tactual sense.