As a separate attachment, it can be nailed to the door as well as the doorframe respectively. After coming back home, the user can directly hang the key on it, so that the ring of the key makes both sides of the attachment fixed, and thus the door cannot be easily opened. However, when the user needs to go out, the door can be opened only after the key is taken down. It not only increases the security but also decreases the probability of forgetting the key in the home.