To reduce the amplitude of an intermodulation signal to be generated by setting inclination to give a curve expressing an amplifier gain as the function of a frequency to be smaller than linear inclination in an area of a low frequency range and to be larger than the inclination of the straight line in an area of a high frequency.

An amplifier 40 and/or an amplifier 60 includes a means for giving positive inclination to a curve expressing the gain of an amplifier or the output level of the amplifier as the function of the frequency such as a filter. The inclination of this curve is set to be smaller than the inclination of a curve obtained in the case of regularly increasing with the frequency being inclination called linear inclination in a low frequency range and is set to be larger at the upper part of the frequency range. In setting a signal level (dBμV unit) to be L, a frequency to be F and a proportion coefficient to be K, it is possible that this curve expressing an amplifier gain with respect to the frequency satisfies L (dBμV)=K*(F)3/2.