A satellite communication system comprising: a communication satellite; a line control device to generate control information transmitted to the communication satellite; and a ground base station (3 - 1 to 3 - 3), the line control device including a control unit to determine, for each of user link signals, which are signals of each of the ground terminals (2-1 to 2-4) transmitted from the ground terminal to the communication satellite, information concerning whether the user link signal is duplicated in the communication satellite and determine a second beam for transmitting a signal to the ground base station (3-1 to 3-3) in a communication satellite allocated to the user link signal and a duplicated signal, which is a signal obtained by duplicating the user link signal, the communication satellite including: a plurality of first antennas (11-2, 11-2) respectively capable of forming different first beams; and a plurality of second antennas (24-1 to 24-3) respectively capable of forming different second beams for transmitting signals to the ground base station (3-1 to 33), the communication satellite including, for each of the first antennas (11-2, 11-2), a demultiplexing unit (16-1 and 16-2) to divide a reception signal received by the first beam into the user link signals, the communication satellite including, for each of the second antennas (24-1 to 24-3), a multiplexing unit (20-1 to 20-3) to multiplex input signals and output the signals to the second antenna (24-1 to 24-3), and the communication satellite including a switch to duplicate the user link signal for which duplication is instructed by the information, generate a duplicated signal, and output the duplicated signal and the user link signal each to different multiplexing units, which correspond to second beams respectively allocated to the duplicated signal and the user link signal, wherein the line control device determines the information on the basis of communication quality requested for the user link signal and transmits the information to the communication satellite. a communication satellite; a line control device to generate control information transmitted to the communication satellite; and a ground base station (3 - 1 to 3 - 3), the line control device including a control unit to determine, for each of user link signals, which are signals of each of the ground terminals (2-1 to 2-4) transmitted from the ground terminal to the communication satellite, information concerning whether the user link signal is duplicated in the communication satellite and determine a second beam for transmitting a signal to the ground base station (3-1 to 3-3) in a communication satellite allocated to the user link signal and a duplicated signal, which is a signal obtained by duplicating the user link signal, the communication satellite including: a plurality of first antennas (11-2, 11-2) respectively capable of forming different first beams; and a plurality of second antennas (24-1 to 24-3) respectively capable of forming different second beams for transmitting signals to the ground base station (3-1 to 33), a plurality of first antennas (11-2, 11-2) respectively capable of forming different first beams; and a plurality of second antennas (24-1 to 24-3) respectively capable of forming different second beams for transmitting signals to the ground base station (3-1 to 33), the communication satellite including, for each of the first antennas (11-2, 11-2), a demultiplexing unit (16-1 and 16-2) to divide a reception signal received by the first beam into the user link signals, the communication satellite including, for each of the second antennas (24-1 to 24-3), a multiplexing unit (20-1 to 20-3) to multiplex input signals and output the signals to the second antenna (24-1 to 24-3), and the communication satellite including a switch to duplicate the user link signal for which duplication is instructed by the information, generate a duplicated signal, and output the duplicated signal and the user link signal each to different multiplexing units, which correspond to second beams respectively allocated to the duplicated signal and the user link signal, wherein the line control device determines the information on the basis of communication quality requested for the user link signal and transmits the information to the communication satellite.