A device network system comprising: a first network including a first device group and a first line group, the first device group including a plurality of devices (101, ···, 106), the first line group being connected to the plurality of devices (101, ···, 106); and a second network including a second device (203, 204) and a second line connected to the second device (203, 204), the second device (203, 204) being physically separated from the first network, characterized in that: in a state in which the first device group is set in advance as a physically connected group or in a state in which the first device group is recognized as a physically connected group, a main device selected from among the plurality of devices (101, ···, 106) is configured to determine, using low-frequency communication, whether the plurality of devices (102, ···, 106) except for the main device belong to the physically connected group, and determine whether a state in which the second device (203, 204) is recognized as belonging to the physically connected group has occurred due to crosstalk between the first network and the second network; and, in a case where the main device detects occurrence of the state in which the second device (203, 204) is recognized as belonging to the physically connected group due to crosstalk between the first network and the second network, the first device group or the second device (203, 204) is configured to cause the second device (203, 204) to leave the physically connected group. a first network including a first device group and a first line group, the first device group including a plurality of devices (101, ···, 106), the first line group being connected to the plurality of devices (101, ···, 106); and a second network including a second device (203, 204) and a second line connected to the second device (203, 204), the second device (203, 204) being physically separated from the first network, characterized in that: in a state in which the first device group is set in advance as a physically connected group or in a state in which the first device group is recognized as a physically connected group, a main device selected from among the plurality of devices (101, ···, 106) is configured to determine, using low-frequency communication, whether the plurality of devices (102, ···, 106) except for the main device belong to the physically connected group, and determine whether a state in which the second device (203, 204) is recognized as belonging to the physically connected group has occurred due to crosstalk between the first network and the second network; and, in a case where the main device detects occurrence of the state in which the second device (203, 204) is recognized as belonging to the physically connected group due to crosstalk between the first network and the second network, the first device group or the second device (203, 204) is configured to cause the second device (203, 204) to leave the physically connected group. in a state in which the first device group is set in advance as a physically connected group or in a state in which the first device group is recognized as a physically connected group, a main device selected from among the plurality of devices (101, ···, 106) is configured to determine, using low-frequency communication, whether the plurality of devices (102, ···, 106) except for the main device belong to the physically connected group, and determine whether a state in which the second device (203, 204) is recognized as belonging to the physically connected group has occurred due to crosstalk between the first network and the second network; and, determine, using low-frequency communication, whether the plurality of devices (102, ···, 106) except for the main device belong to the physically connected group, and determine whether a state in which the second device (203, 204) is recognized as belonging to the physically connected group has occurred due to crosstalk between the first network and the second network; and, in a case where the main device detects occurrence of the state in which the second device (203, 204) is recognized as belonging to the physically connected group due to crosstalk between the first network and the second network, the first device group or the second device (203, 204) is configured to cause the second device (203, 204) to leave the physically connected group.