PURPOSE: To easily measure a second order partial differential coefficient regarding a plurality of variables by adding modulation signals having different frequencies to dependent variables of a signal to be measured and by multiplying obtained signals by an alternate signal of frequencies equivalent to a difference or sum of two modulation signals.

CONSTITUTION: A signal to be measured entering from an input terminal 1 enters an adder 6 after extra oscillating components are filtered off. A rectangular wave having a frequency equivalent to a difference or sum of frequencies of two modulation signals created by a rectangular wave generating circuit 5 enters the adder 6 and multiplied by a signal to be measured that is the output of the filter 4. As a result of multiplication, a signal outputted from the adder 6 can be regarded as the sum of a dc signal and an ac signal. The do signal is proportional to a second order partial differential coefficient and is therefore passed through a low-frequency pass filter 7 and amplified by time average, thereby selectively amplifying the second order partial differential coefficient.