An AD converter includes an analog data storing unit, a first DA converter for converting an input digital data into a first analog reference voltage which varies within a first voltage range in a range of every possible signal voltage of the input analog data, a second DA converter for converting the input digital data into a second analog reference voltage which varies within a second voltage range in the range of every possible signal voltage of the input analog data, a first comparator for comparing the input analog data with the first reference voltage, a second comparator for comparing the input analog data with the second reference voltage and a digital data storing unit for storing a digital data corresponding to a point of time when a change of state occurs in the comparison results of each of the first and second comparators.