An A/D converter circuit uses first and second ring delay lines. The first and second ring delay lines are supplied with input signals, which increase/decrease oppositely from each other with respect to change directions. In each ring delay line, a first counter counts the number of times of circulation of a pulse signal circulating therein to find a digital data, and a last digital data is subtracted from a present digital data. By adding the resulting first and second digital data of the first and second ring delay lines, a digital data of the input voltage of linear characteristics is provided.