A power supply modulation device comprising: a detecting unit (12, 50) to detect a first amplitude from a first digital signal and detect a second amplitude from a second digital signal, the first amplitude being an amplitude of a first analog signal provided to a first amplifier, and the second amplitude being an amplitude of a second analog signal provided to a second amplifier; a load modulation determining unit configured to calculate a time differential value of a ratio of the first amplitude detected by the detecting unit to a sum of the first amplitude and the second amplitude detected by the detecting unit, and perform determination of, on a basis of the time differential value of the ratio, whether or not output impedance of a combining circuit changes along with a change in power of the combined signal obtained by the combining circuit, wherein the combining circuit is configured to obtain a combined signal by combining together the first analog signal amplified by the first amplifier and the second analog signal amplified by the second amplifier, wherein if a time differential value of the ratio is 0 the load modulation determining unit determines that the output impedance of the combining circuit does not change and that load modulation does not occur, and if a time differential value of the ratio is not 0 the load modulation determining unit determines that the output impedance of the combining circuit changes and that load modulation occurs; and a power supply voltage control unit to control a power supply voltage supplied to each of the first amplifier and the second amplifier, on a basis of a result of the determination by the load modulation determining unit; wherein when the load modulation determining unit determines that the output impedance changes, the power supply voltage control unit fixes the power supply voltage supplied to each of the first amplifier and the second amplifier, and when the load modulation determining unit determines that the output impedance does not change, the power supply voltage control unit controls the power supply voltage supplied to each of the first amplifier and the second amplifier, in accordance with a greater one of the first amplitude and the second amplitude. a detecting unit (12, 50) to detect a first amplitude from a first digital signal and detect a second amplitude from a second digital signal, the first amplitude being an amplitude of a first analog signal provided to a first amplifier, and the second amplitude being an amplitude of a second analog signal provided to a second amplifier; a load modulation determining unit configured to calculate a time differential value of a ratio of the first amplitude detected by the detecting unit to a sum of the first amplitude and the second amplitude detected by the detecting unit, and perform determination of, on a basis of the time differential value of the ratio, whether or not output impedance of a combining circuit changes along with a change in power of the combined signal obtained by the combining circuit, wherein the combining circuit is configured to obtain a combined signal by combining together the first analog signal amplified by the first amplifier and the second analog signal amplified by the second amplifier, wherein if a time differential value of the ratio is 0 the load modulation determining unit determines that the output impedance of the combining circuit does not change and that load modulation does not occur, and if a time differential value of the ratio is not 0 the load modulation determining unit determines that the output impedance of the combining circuit changes and that load modulation occurs; and a power supply voltage control unit to control a power supply voltage supplied to each of the first amplifier and the second amplifier, on a basis of a result of the determination by the load modulation determining unit; wherein when the load modulation determining unit determines that the output impedance changes, the power supply voltage control unit fixes the power supply voltage supplied to each of the first amplifier and the second amplifier, and when the load modulation determining unit determines that the output impedance does not change, the power supply voltage control unit controls the power supply voltage supplied to each of the first amplifier and the second amplifier, in accordance with a greater one of the first amplitude and the second amplitude.