A communication system comprising: a plurality of transmission apparatuses (103, 105) for terminating a respective plurality of communication lines (1,2), including a first transmission apparatus that terminates a first communication line; and a second transmission apparatus that terminates a second communication line; and a line control apparatus that performs data communication with the plurality of transmission apparatuses including the first and second transmission apparatuses (103, 105); wherein the first transmission apparatus is configured to monitor a line state of the first communication line and notify the line control apparatus of the line state of the first communication line by using a first control packet, the line state being set in the first control packet, the second transmission apparatus is configured to monitor a line state of the second communication line and notify the line control apparatus of the line state of the second communication line by using a second control packet, the line state being set in the second control packet, and the line control apparatus is configured to: set a band setting value based on the total transmission band of the first and second communication lines (1,2) after a change of modulation method, perform band control in accordance with the band setting value, and transmit data to the plurality of transmission apparatuses including the first and second transmission apparatuses (103, 105), when the change of modulation method of at least one of the first and second communication lines (1, 2) is detected. a plurality of transmission apparatuses (103, 105) for terminating a respective plurality of communication lines (1,2), including a first transmission apparatus that terminates a first communication line; and a second transmission apparatus that terminates a second communication line; and a line control apparatus that performs data communication with the plurality of transmission apparatuses including the first and second transmission apparatuses (103, 105); wherein the first transmission apparatus is configured to monitor a line state of the first communication line and notify the line control apparatus of the line state of the first communication line by using a first control packet, the line state being set in the first control packet, the second transmission apparatus is configured to monitor a line state of the second communication line and notify the line control apparatus of the line state of the second communication line by using a second control packet, the line state being set in the second control packet, and the line control apparatus is configured to: set a band setting value based on the total transmission band of the first and second communication lines (1,2) after a change of modulation method, perform band control in accordance with the band setting value, and transmit data to the plurality of transmission apparatuses including the first and second transmission apparatuses (103, 105), when the change of modulation method of at least one of the first and second communication lines (1, 2) is detected. set a band setting value based on the total transmission band of the first and second communication lines (1,2) after a change of modulation method, perform band control in accordance with the band setting value, and transmit data to the plurality of transmission apparatuses including the first and second transmission apparatuses (103, 105), when the change of modulation method of at least one of the first and second communication lines (1, 2) is detected.