To effectively compensate distortion occurring in a second-order harmonic injection amplifier, in a predistorter for compensating for the distortions that occur in a second-order harmonic injection amplifier.

In the predistorter (for instance, a memory-less PD 31a) for compensating for the distortions occurring in a second-order harmonic injection amplifier, an amplitude value detection means 41a (a fundamental wave amplitude detection part 51, a second-order harmonic amplitude detection part 52, a multiplier 53, an adder 54) detects the value of the result of adding amplitude, based on an input signal to amplitude based on its second-order harmonic signal; and a distortion compensation process execution means (a distortion compensation table 42, a PD executing part 43) executes processes for compensating for the distortion occurring in the second-order harmonic injection amplifier, based on the values detected by the amplitude value detection means 41a.