To provide a network controlling method capable of suppressing a load increase of a network even when a shared device needing many data in a computer network undergoing a connectable leased line connection through a public line.

Whether or not a public line is included in a path between a shared input-output device and a computer system using it, is detected (step S502), and capability information that is equal to or less than the original capability of the shared input-output device is returned to a device requesting use when it is included (step S503). Thus, the computer system using the shared input-output device can suppress the settable range of an operation condition affecting data quantity such as settable resolution and the number of gradations.