PURPOSE: To obtain a new serial type A/D conversion system by repeating subtraction based upon voltage having a specific ratio to input analog voltage and logically converting the judged results of polarity in respective subtracted results.

CONSTITUTION: A subtractor 111 subtracts inputted analog voltage Vin from 2-1Vref. The Vref is reference voltage corresponding to the full scale of the voltage Vin. When output voltage Vout1 obtained as the result of subtraction is used as it is when the value is positive or converted into a positive value in the case of a negative value and the positive value is inputted to the succeeding subtractor 112 and subtracted from 2-2Vref. The operation is repeated by n-bit times, Vout 1 to Voutn obtained by respective subtracted results are respectively compared with a zero cross by comparators 121 to 12n and outputs a1 to an which are '1' when the polarity is positive or '0' in the case of negative polarity are obtained. The outputs a1 to an are encoded by an encoder 14 to obtain digital outputs b1 to bn.