A network system includes: a network device; and a portable connection device capable of connecting thereto a terminal device which accesses the network device, where the portable connection device holds authentication information related to the connection between the network device and the terminal device, the authentication information being previously set. Here, the network device includes a processor, and the processor acquires the authentication information held in the portable connection device according to the connection of the portable connection device to the network device, acquires identification information on the terminal device from the terminal device according to the connection of the terminal device to the portable connection device, and compares the acquired authentication information with the identification information on the terminal device and device information on the network device to determine whether or not the access from the terminal device is allowed.