An IP address distribution system for distributing an IP address to a client device connected to an IP network based on distribution request information received from the client device, comprises: a lease condition storage unit that stores lease conditions corresponding to the client device indicating conditions relating to approval/prohibition of IP address distribution; an initial control unit that, when the distribution request information is received from the client device, approves the IP address distribution to the client device and stores initial lease conditions corresponding to the client device into lease condition storage unit, if the lease conditions corresponding to the client device are not stored in the lease condition storage unit; a condition modification unit that modifies the lease conditions corresponding to the client device stored in the lease condition storage unit; and an IP address distribution approval/prohibition control unit that controls approval/prohibition of IP address distribution to the client device as a transmission source of the distribution request information, based on the lease conditions corresponding to the client device stored in the lease condition storage unit.