To provide an analog/digital(A/D) converting method, based on a spike for a current mode.

A count-down circuit for an A/D converter, based on the spike for the current mode, is composed of first and second neurons, with which each neuron has an analog state variable, the first neuron has an unknown angle displacement -θ as an initial state and the second neuron has a know angle displacement '0' as an initial state, and a means for supplying currents respectively to the first and second neurons in the ratio of 1:R (R is an integer) until the first neuron generates a first spike output. Thus, an approximate value, corresponding to the initial state of the first neuron is given by nπ/2 ((n), is the number of spike outputs from the second neuron), the state of the second neuron at the time of first spike output is the remainder expressing the error of the approximate value, and further this circuit is composed of a precise means for providing the more proximate approximate value of the initial state by more precisely evaluating the remainder.