1. A data relay apparatus, comprising: a first demodulating unit, for demodulating a modulation signal from a first transmission source, modulation mode of the modulation signal from the first transmission source being a first modulation mode; a second demodulating unit, for demodulating a modulation signal from a second transmission source, the modulation mode of the modulation signal from the second transmission source being a second modulation mode with a higher modulation order than the first modulation mode; a combination controlling unit, for providing mapping rule necessary for performing signal combination in accordance with the modulation mode of the modulation signal from the first transmission source and the modulation mode of the modulation signal from the second transmission source, the mapping rule being rule as how to map the demodulated modulation signal from the first transmission source together with the demodulated modulation signal from the second transmission source to a constellation of symbols of the second modulation mode; and a data combining unit, for performing data combination on the demodulated modulation signal from the first transmission source and the demodulated modulation signal from the second transmission source in accordance with the mapping rule provided by the combination controlling unit.