1. A signal processing apparatus having a circuit network formed by connecting a plurality of neuron units into a network, said neuron unit being provided with self-learning means which comprises: weight function varying means for varying a weight function of neurons; and weight function generating means which generates a variable weight function for the weight function varying means, on the basis of a positive or negative error signal obtained as a result of the comparison between an output signal and a teaching signal, said signal processing apparatus being provided with differential coefficient calculating means for calculating two kinds of differential coefficients of a neuron response function, on the basis of an output signal from neuron units so that said positive and negative error signals can be generated.