Abstract: An input unit enters two reference signals having respective different frequencies at a stage preceding a power amplifier. The reference signals are delivered through a predistortion circuit to a power amplifier, and supplied from a predetermined section of the power amplifier. The signals from the power amplifier contain a third-order intermodulation distortion component. Based on the signals from the power amplifier, an extracting unit extracts a frequency component having a frequency which is twice the difference between the frequencies of said two reference signals. A detecting unit detects the extracted frequency component thereby to output a DC voltage. Since the extracted frequency component is correlated to the third-order intermodulation distortion component, the DC voltage represents the magnitude of the third-order inter-modulation distortion component.