1. A learning method of a neural network circuit including a plurality of neural network circuit elements which are interconnected, wherein each of the plurality of neural network circuit elements includes: at least one synapse circuit which receives an input a signal, as a first input signal, output from another neural network circuit element, the first input signal having a bipolar sawtooth pulse waveform; and one neuron circuit which receives as an input a signal output from the at least one synapse circuit, wherein the synapse circuit includes a variable resistance element which includes: a semiconductive layer having a first electrode and a second electrode; a ferroelectric layer formed on a main surface of the semiconductive layer; and a control electrode formed on the ferroelectric layer, a resistance value between the first electrode and the second electrode changing in response to an electric potential difference between the first electrode and the control electrode, wherein the synapse circuit is configured to perform switching between a state in which the first input signal is permitted to be input to the first electrode of the variable resistance element, and a state in which the first input signal is inhibited from being input to the first electrode, wherein the neuron circuit includes a waveform generating circuit for generating a bipolar sawtooth pulse voltage, wherein the method comprises: for a period during which the first input signal from another neural network circuit element is permitted to be input to the first electrode of the variable resistance element, inputting the bipolar sawtooth pulse voltage to the control electrode of the variable resistance element, the bipolar sawtooth pulse voltage being generated within the neural network circuit element including the variable resistance element, and wherein the resistance value of the variable resistance element changes due to an electric potential difference between the first electrode and the control electrode, the electric potential difference being generated depending on an input timing difference between a voltage applied to the first electrode and the voltage applied to the control electrode.