When a PC establishes a communication via the Internet with another external PC, at least one of investigation methods is executed prior to a commencement of the communication in a system provided by this invention. The methods are an IP address investigation for investigating whether it is global or private; an IP address comparison for comparing an IP address recognized by an external terminal with the IP address which has been allocated to the own terminal; and a port-scanning judgement for judging as to the PC can access the external PC by executing a port-scanning with respect to a port used by the communication application program from the external terminal. By those methods, when there is a possibility that the communication application program cannot be used due to a certain reason of the investigated network environment, this fact is displayed on a screen so as to be notified to the user.