A network system comprises first to third routers coupled in series. The first router transmits a first packet indicating a first priority or a second packet indicating a second priority higher than the first priority. The second router receives the first packet, determines whether or not the first priority of the first packet is to be changed or not on the basis of both a first bandwidth which is assigned for receiving, from the first router, a second packet which is indicating the second priority and a second bandwidth which is assigned for communicating a packet including the first packet and the second packet, changes the priority of the first packet if it is decided that its priority is to be changed, and transmits to the third router the packet of which the priority has been changed to the second priority from the first priority. The third router receives the packet transmitted from the second router.