An intercom system that includes an electronic lock, an entrance slave device, an intercom master device that connects to the electronic lock and the entrance slave device in such a manner that communication with the electronic lock and the entrance slave device is possible, and an authentication server that connects to the intercom master device in such a manner that communication with the intercom master device is possible, wherein the authentication server is configured to perform an exchange of second key information with the intercom master device and to establish a first encryption communication path between the authentication server itself and the intercom master device, wherein the authentication server is configured to perform an exchange of fourth key information with a wireless terminal and to establish a second encryption communication path between the authentication server itself and the wireless terminal, the wireless terminal being configured to perform communication with the intercom master device through the authentication server, wherein the intercom master device is configured to perform an exchange of first key information and third key information with the wireless terminal and to establish a third encryption communication path between the intercom master device itself and the wireless terminal, and characterized in that the intercom master device is configured to receive an instruction to unlock the electronic lock, which is transmitted through the first encryption communication path, the second encryption communication path, and the third encryption communication path, from the wireless terminal, wherein a double encryption communication path is established between the intercom master device and the authentication server and between the authentication server and the wireless terminal such that data transmitted from the intercom master device through the authentication server towards the wireless terminal is encrypted using the first key information and the second key information, and data transmitted from the wireless terminal through the authentication server towards the intercom master device is encrypted using the third key information and the fourth key information, wherein, when it is detected that a person came to an entrance door, the entrance slave device transmits a visiting notification to the intercom master device, and the intercom master device starts a time tracking of the time that has elapsed after the visiting notification was received, and wherein at a time of receiving the instruction to unlock the electronic lock, the intercom master device compares the tracked time with an operation performance time that is prescribed in advance as a time length during which the electronic lock has to be unlocked with a remote operation from the wireless terminal, and causes the electronic lock to be unlocked in case the tracked time is shorter than the operation performance time. wherein the authentication server is configured to perform an exchange of second key information with the intercom master device and to establish a first encryption communication path between the authentication server itself and the intercom master device, wherein the authentication server is configured to perform an exchange of fourth key information with a wireless terminal and to establish a second encryption communication path between the authentication server itself and the wireless terminal, the wireless terminal being configured to perform communication with the intercom master device through the authentication server, wherein the intercom master device is configured to perform an exchange of first key information and third key information with the wireless terminal and to establish a third encryption communication path between the intercom master device itself and the wireless terminal, and characterized in that the intercom master device is configured to receive an instruction to unlock the electronic lock, which is transmitted through the first encryption communication path, the second encryption communication path, and the third encryption communication path, from the wireless terminal, wherein a double encryption communication path is established between the intercom master device and the authentication server and between the authentication server and the wireless terminal such that data transmitted from the intercom master device through the authentication server towards the wireless terminal is encrypted using the first key information and the second key information, and data transmitted from the wireless terminal through the authentication server towards the intercom master device is encrypted using the third key information and the fourth key information, wherein, when it is detected that a person came to an entrance door, the entrance slave device transmits a visiting notification to the intercom master device, and the intercom master device starts a time tracking of the time that has elapsed after the visiting notification was received, and wherein at a time of receiving the instruction to unlock the electronic lock, the intercom master device compares the tracked time with an operation performance time that is prescribed in advance as a time length during which the electronic lock has to be unlocked with a remote operation from the wireless terminal, and causes the electronic lock to be unlocked in case the tracked time is shorter than the operation performance time.