A port number identification system, comprising: an administration server which includes an exclusive port that is a well-known port used for transmitting and receiving a file and a plurality of multi-purpose ports, other than the well-known port, with different port numbers and individually accepts connection from outside via each of the multi-purpose ports; and a plurality of administrated machines which perform communication by designating one of the multi-purpose ports as a connection destination, wherein the administration server comprises: administration server side storage module for storing a port number file containing a port number of a first multi-purpose port; and file server means for taking out the port number file from the administration server side storage module in response to a request for the port number file made from the administrated machine via the exclusive port, and transmitting the port number file to the administrated machine via the exclusive port, wherein each of the administrated machines comprises: file acquisition means for requesting the port number file from the administration server via the exclusive port and acquiring the port number file transmitted via the exclusive port; and port number identification means for judging whether or not a connection via a second multi-purpose port can be done, directing the file acquisition means to acquire the port number file when it is judged that the connection via the second multi-purpose port cannot be done, identifying the port number of the first multi-purpose port based on the port number file acquired by the file acquisition means via the exclusive port, and using the port number of the first multi-purpose port as a connection destination to the administration server. administration server side storage module for storing a port number file containing a port number of a first multi-purpose port; and file server means for taking out the port number file from the administration server side storage module in response to a request for the port number file made from the administrated machine via the exclusive port, and transmitting the port number file to the administrated machine via the exclusive port, wherein each of the administrated machines comprises: file acquisition means for requesting the port number file from the administration server via the exclusive port and acquiring the port number file transmitted via the exclusive port; and port number identification means for judging whether or not a connection via a second multi-purpose port can be done, directing the file acquisition means to acquire the port number file when it is judged that the connection via the second multi-purpose port cannot be done, identifying the port number of the first multi-purpose port based on the port number file acquired by the file acquisition means via the exclusive port, and using the port number of the first multi-purpose port as a connection destination to the administration server. file acquisition means for requesting the port number file from the administration server via the exclusive port and acquiring the port number file transmitted via the exclusive port; and port number identification means for judging whether or not a connection via a second multi-purpose port can be done, directing the file acquisition means to acquire the port number file when it is judged that the connection via the second multi-purpose port cannot be done, identifying the port number of the first multi-purpose port based on the port number file acquired by the file acquisition means via the exclusive port, and using the port number of the first multi-purpose port as a connection destination to the administration server.