The purpose of the present invention is to compensate the distortion of an amplifier by a predistorter which compensates the distortion of a power amplifier even when a frequency amplitude deviation exists. The predistorter comprises a distortion compensation coefficient storage unit for storing a distortion compensation coefficient corresponding to the reference value using the characteristic quantity of the envelope of a transmission signal before being inputted to the power amplifier as a reference value; and a reference value conversion unit for converting the characteristic quantity into a predetermined value according to the size of the characteristic quantity, and when the characteristic quantity is, for example, the amplitude value of an envelope and the amplitude value is small, that value is rounded up, for example, to an upper-limit value within a linear range of the amplifier.