To overcome the disadvantage in security of the conventional door chain device in which the length for a chain is determined to keep the width of an opening part of a door to 10 cm, one end of the chain is fixed, and the opposite end part is removable to allow going in and out the door, that is, to solve the problem of the conventional chain fixing method that screws are simply removed by a screwdriver because brackets at both ends are fixed to a door and a building by wood screws, so that when a concerned person is not present near the inside of the door during the night, and the door is not locked, or illegally unlocked by picking, even if the door chain is latched, the opening is formed for the length of the chain so that a burglar inserts his hand to remove the screws of the brackets and breaks into the interior of the building without being noticed.

The above problem is solved by constructing the door chain device with a controllable length of the chain. When the length of the chain is set shorter, even if the door is unlocked illegally, the hand can not be inserted so that the chain is not removed to prevent break-in. The length of the chain may be arbitrarily extended shorter.