In accordance with another representative embodiment, a high-frequency signal transmitter a power amplifier configured to supply a high-frequency signal; an antenna configured to transmit the high-frequency signal; a transmission line configured to transfer the high-frequency signal from the power amplifier to the antenna; and an impedance matching circuit connected to the transmission line. The high-frequency signal transmitter also comprises a mismatch detector. The mismatch detector is configured to designate a comparatively poor linearity region and a comparatively good linearity region by dividing a Smith chart into the two regions based on Adjacent Channel Power Ratio (ACPR) contours drawn on the Smith chart at a point on the transmission line where the impedance matching circuit is connected, to measure a time-dependent reflection coefficient of the high-frequency signal transmitter in terms of a phase and a magnitude, to determine whether the reflection coefficient is located in the comparatively poor linearity region or the comparatively good linearity region, and based on a result of the determination, to improve the linearity of the high-frequency signal transmitter.