A network system comprising a plurality ofnetwork devices (101, 102, 103, 104, 105, 105) that transfer frames by repeating, in a constant cycle, a reserved transfer interval that is a time band, in which a frame is transferred with a reservation, and a free transfer interval that is a time band, in which a frame is freely transferred; a first network device that transmits a frame; a second network device that is a transmission destination of the frame; and a relay network device, which links the first network device and the second network device, characterized in that: the first network device transmits a first pre-reserved frame for reserving the frame to the relay network device, the relay network device determines whether the frame reservation is possible by checking a reservation status which is stored within the relay network device, and if the frame reservation is determined to be possible the relay network device produces, based on the first pre-reserved frame, a second pre-reserved frame and transmits the second pre-reserved frame to the second network device, the second network device determines whether the frame reservation is possible by checking a reservation status which is stored within the second network device, and if the frame reservation is determined to be possible the second network device produces, based on the second pre-reserved frame, a reservation check frame and transmits the reservation check frame to the relay network device, the first pre-reserved frame includes a reservation start time A and a reservation end time B in a predetermined communication cycle, and a required time band C, the relay network device determines whether a free time band that is equal to or greater than the required time band C is present within a time band from the reservation start time A to the reservation end time B in the predetermined communication cycle, and produces and transmits the second pre-reserved frame that includes a reservation start time D and a reservation end time E within the free time band and the required time band C when it is determined that the free time band is present, the second network device determines whether a free time band that is equal to or greater than the required time band C is present within a time band from the reservation start time D to the reservation end time E in the predetermined communication cycle, and produces and transmits the reservation check frame that includes a reservation registration start time and a reservation registration end time within the free time band when it is determined that the free time band is present, and the first network device and the relay network device determine a reserved time band within the predetermined communication cycle based on the reservation registration start time and the reservation registration end time that are included in the reservation check frame. the first network device transmits a first pre-reserved frame for reserving the frame to the relay network device, the relay network device determines whether the frame reservation is possible by checking a reservation status which is stored within the relay network device, and if the frame reservation is determined to be possible the relay network device produces, based on the first pre-reserved frame, a second pre-reserved frame and transmits the second pre-reserved frame to the second network device, the second network device determines whether the frame reservation is possible by checking a reservation status which is stored within the second network device, and if the frame reservation is determined to be possible the second network device produces, based on the second pre-reserved frame, a reservation check frame and transmits the reservation check frame to the relay network device, the first pre-reserved frame includes a reservation start time A and a reservation end time B in a predetermined communication cycle, and a required time band C, the relay network device determines whether a free time band that is equal to or greater than the required time band C is present within a time band from the reservation start time A to the reservation end time B in the predetermined communication cycle, and produces and transmits the second pre-reserved frame that includes a reservation start time D and a reservation end time E within the free time band and the required time band C when it is determined that the free time band is present, the second network device determines whether a free time band that is equal to or greater than the required time band C is present within a time band from the reservation start time D to the reservation end time E in the predetermined communication cycle, and produces and transmits the reservation check frame that includes a reservation registration start time and a reservation registration end time within the free time band when it is determined that the free time band is present, and the first network device and the relay network device determine a reserved time band within the predetermined communication cycle based on the reservation registration start time and the reservation registration end time that are included in the reservation check frame.