Abstract: For preventing an analog-to-digital converting unit for a successive approximation from error in operation, the analog-to-digital converting unit has not only a first comparator circuit but also second and third comparator circuits for comparing an analog input signal with first and second critical signals indicative of a responsible voltage range, and the second or third comparator circuit produces a first or second warning signal in the presence of the analog input signal output the responsible voltage range, then a sequential approximation controlling circuit restricts a bit signal of an digital output signal thereof which allows a digital-to-analog converting circuit to produce a comparison voltage signal within a predetermined range, thereby preventing the first comparator circuit from a error in operation.