A modulator 23 corrects an amplitude signal Rd by adding thereto an offset value α and generates, based on the corrected amplitude signal Rd, an amplitude signal Ra for amplitude-modulating a radio-frequency signal. The modulator 23 includes: an offset voltage measurement section 109 operable to measure an offset voltage V2cal of the amplitude signal Ra; a correction value calculation section 110 operable to obtain a difference value ΔV2cal between the offset voltage V2cal measured by the offset voltage measurement section 109 and an initial value of the offset voltage V2cal and operable to calculate, based on the difference value ΔV2cal, a correction value VODAC for correcting the offset voltage V2cal; and an addition section 106 operable to add the correction value VODAC calculated by the correction value calculation section 110 to a signal processed until the amplitude signal Ra is generated from the amplitude signal Rd.