An imaging system includes an A/D converter including a holding unit holding a pixel signal as a voltage level, a comparator comparing the voltage level held with a reference level, a circuit capable of changing the voltage level so as to approach the reference level at first and second rates, wherein the voltage level is changed at the first rate to determine higher bits in accordance with inversion of a relationship between the reference level and the voltage level, after that, the voltage level is changed at the second rate to determine lower bits in accordance with inversion of the relationship between the reference level and the voltage level, and an adjusting unit which adjusts the voltage level during a period until the voltage level is changed at the second rate after determination of the higher bits so that the lower bits and the voltage level hold a linear relationship.