To provide a door unlocking device which allows a user and the managing agent to release a lock of a rented villa at any time, and also allows them to release the lock even if system failure or the like has occurred, and to provide a lock managing system for the rented villa by using the unlocking device.

According to the lock managing system, a first memory 12 of the door unlocking device 10 has stored therein a managing dealer password beforehand. A second memory 14 has stored therein a user password that is given to the user who has reserved the rented villa. The user password is transmitted from a lock managing agent server 26 to the door unlocking device 10 at a check-in time on a reservation period starting date, and stored in the second memory 14. Thus the user can release the door lock by inputting the user password of himself or herself in a password input device 16.