In order to more efficiently use port resources, which are finite global address resources assigned to an address translation device, the address translation device holds a session-port assignment table showing a correspondence between an existing session and a local endpoint (port resource) in the address translation device, and a port assignment rule indicating port usage about assignable ports. An address translation unit translates address information of a packet received according to the correspondence between the existing session and the port resource shown in the session-port assignment table, and assigns the port according to the port usage indicated by the port assignment rule for a packet for opening a new session. An assignment rule update unit changes a ratio of the port usage in the port assignment rule while the correspondence between the existing session and the port resource in the session-port assignment table is not changed.