To provide a network address setting method and a network address setting dongle to set a network address of a computer system 50 and a peripheral equipment 21 in a simple manner by using a dongle 1.

When it is decided that the network address A of the peripheral equipment 21 is included in the network address E connected by the computer system 50, the dongle 1 adds a postscript of the network address E to a non-volatile storage portion of the dongle 1 as a non-permissible network address; generates the network address B other than the non-permissible network address, including the network address E, sets the network address B in the peripheral equipment 21; adds the postscript of the network address B to the non-volatile storage portion of the dongle; and sets the network address B which is not included in the network address E in the computer system 50.