An IP-PBX has a software-based built-in control unit, a card slot into which a control card is plugged, and a data bus connecting the software-based control unit and the card slot. The software-based control unit includes a first management unit managing up to a first number of IP terminals, and an IP terminal registering unit connected to the first management unit and the card slot. The control card includes a second management unit managing up to a second number of IP terminals. The IP terminal registering unit compares a current number of IP terminals managed by the first management unit with the first number when an IP terminal requests communication. The IP terminal has an ID number and an IP address. If the current number is smaller than the first number, the IP terminal registering unit associates the first management unit with the ID number and the IP address, and the first management unit manages the IP terminal. If the current number is equal to the first number, the IP terminal registering unit associates the second management unit with the ID number and the IP address, and the second management unit manages the IP terminal.