An upper comparator group compares an analog signal with upper reference potentials applied from upper ladder resistance network. A switch group outputs the predetermined intermediate reference potential of the ladder resistance network to an analog subtracting circuit in response to the upper comparison results. The analog subtracting circuit subtracts the intermediate reference potential from the analog signal for producing an input signal for use in the lower side. A lower ladder resistance network outputs lower reference potentials obtained by dividing by resistors constant static intermediate reference potentials of the ladder resistance network applied from a differential amplifying circuit. A lower comparator group compares the lower reference potentials with the input signal for lower comparison. The upper and the lower comparison results are converted into a digital signal by upper and the lower encoders and the adding/subtracting circuit.