A network system of a projector is provided in which a network connection is established between a plurality of information terminal apparatus and the projector, and an image data file owned by an information terminal apparatus can be commonly used. A network system of a projector is arranged by an information terminal apparatus and a projector connectable to the information terminal apparatus via a network. The information terminal apparatus is arranged by a personal computer which supplies image data to the projector, and personal computers which are connected to this personal computer by a wireless manner. When the network connection is established between the personal computer and the projector, and when the network connection between the personal computer and the personal computers is established, the image data is commonly used among these personal computers.