A virtual private network connecting system in a virtual private network is formed by accommodating in a public network a plurality of PBX units each assigned a unique private office number (ON), the system executed by the units comprising in each of the PBX unit. A virtual private network connecting system comprises number converting units for converting a private office number (ON) to a subscriber number (SN) assigned to the destination PBX unit by the public network and identifying the system of terminating connecting a call from the public network to the destination PBX unit when an extension accommodated in the PBX unit issues a call and dials the private office number (ON) assigned to the destination PBX unit, and outgoing call control units for controlling communication between the called extension accommodated in the destination PBX unit through the public network according to the number conversion result provided by the number converting units and the identification of the system of terminating connection.