An authentication apparatus, comprising: a registration unit configured to register authentication information of authorized users; a first authentication unit configured to authenticate a user at an entrance of the user to a first area; a second authentication unit configured to authenticate a user at an entrance of the user to a second area after having been authenticated by the first authentication unit; and a person tracking device configured to track the authenticated users in the first area and to detect a user present in a detection area and expected to be authenticated by the second authentication unit, the detection area being set as a predetermined area in the vicinity of the entrance to the second area, wherein the second authentication unit is configured to retrieve, from the registration unit, authentication information of the user detected by the person tracking device as being present within the detection area and to authenticate the user already authenticated by the first authentication unit using the retrieved authentication information, characterized in that the person tracking device is further configured to detect, in accordance with tracking information of the person tracking device, that a user has gotten close to or in physical contact with another user in the first area, and wherein the second authentication unit is further configured to retrieve, from the registration unit, authentication information of the other user that the user has gotten close to or in physical contact with, together with the registration information of the user detected in the detection area, and to authenticate the detected user using both the retrieved authentication information. a registration unit configured to register authentication information of authorized users; a first authentication unit configured to authenticate a user at an entrance of the user to a first area; a second authentication unit configured to authenticate a user at an entrance of the user to a second area after having been authenticated by the first authentication unit; and a person tracking device configured to track the authenticated users in the first area and to detect a user present in a detection area and expected to be authenticated by the second authentication unit, the detection area being set as a predetermined area in the vicinity of the entrance to the second area, wherein the second authentication unit is configured to retrieve, from the registration unit, authentication information of the user detected by the person tracking device as being present within the detection area and to authenticate the user already authenticated by the first authentication unit using the retrieved authentication information, characterized in that the person tracking device is further configured to detect, in accordance with tracking information of the person tracking device, that a user has gotten close to or in physical contact with another user in the first area, and wherein the second authentication unit is further configured to retrieve, from the registration unit, authentication information of the other user that the user has gotten close to or in physical contact with, together with the registration information of the user detected in the detection area, and to authenticate the detected user using both the retrieved authentication information.