To compute the higher-order strain component of mutual conductance by inputing sine wave superimposed with DC current to FET as gate voltage, and analyzing a digital voltage signal converted from drain output current.

When sine wave from a signal source 3 superimposed with DC current by a power supply 4 for bias is input to FET 1, drain current is outputted from the FET 1. The drain current is amplified, converted to a voltage signal by a current/voltage converter 6, the signal level is regulated by a level regulater 8, and input to an A/D converter 7. The input signal to the A/D converter 7 is sampled with a cycle so as to be divided into 2N (N is an integer, N >7 is desirable) for the input sine wave frequency to the FET 1, and converted to a digital signal. The digital signal is input to a personal computer 9 to be analyzed through Fourier transform. As a result of this, the voltage signal input to the A/D converter 7 can detect not only the basic wave component, but the higher-order strain component.