There has been a problem of necessitating a complicated work for establishing a connection capable of intercommunication between a computer as server and a projector as client.When a connection is established between a server 20 and projectors 301 through 30n, a communication command is issued according to UDP-based broadcasting to retrieve connectable projectors 301 through 303. Thereafter, the projector 301 to be connected is selected and is assigned an IP address to establish the connection. In this manner, a user can easily obtain a connection environment capable of intercommunication between the server 20 and the projector 301. A TCP/IP communication command is used to input or output image data, ensuring reliable transmission. Since the authentication can be provided using a keyword projected during the connection establishment, it is possible to prevent an unauthorized access to the projector 301 by an outsider.