1. A nonlinear operation unit, comprising: nonlinear function operation means for performing the computing operation of a function, which produces the results of operations equivalent to the results of the process carried out by computing at least one input signal by using a nonlinear function, and multiplying the results of the computing operations by weight values; and adder means for adding together the results of the computing operations by said nonlinear function operation means and adding a threshold value to the sum of these results; wherein said nonlinear function operation means includes differential amplifier circuits, each said differential amplifier circuit consisting of a pair of amplifier elements each having at least three terminals, said paired amplifier elements being coupled with each other at the first terminals, a voltage difference proportional to the input signal is applied to between the second terminals of said paired amplifier elements each having at least three terminals, a current proportional to the weight is fed to the first terminals, whereby the output signal is produced as a current difference at the third terminals of the amplifier elements each having at least three terminals and constituting differential amplifier circuits.