PURPOSE: To attain high resolution with high accuracy and high stability by summing bit outputs of plural A/D converters so as to obtain a digital conversion output from an analog input voltage.

CONSTITUTION: A level detection section 101 detects that to which range of a reference voltage a voltage of an analog input signal stays and provides an output of a 2-bit parallel signal to a reference power supply section 102 to select reference voltages Vref (TOP), (BOTTOM) to A/D converters 103, 104. Then the reference voltage for the converter 104 is set in the initial state and the converters 103, 104 convert the input signal. The converter 103 provides an output of 2-bits and the converter 104 provides an output of 8-bits in total 10-bits. Thus, troublesome adjustment for selecting the reference voltage inputted to the A/D converters 103, 104 in terms of an analog signal level is not required and highly stable high resolution is obtained.