PURPOSE: To simplify the structure and to attain high speed processing by supplying a prescribed reference voltage at any time based on the conversion data up to the pre-stage to a comparator for each bit.

CONSTITUTION: A sample-and-hold circuit S sampling and holding a signal with each bit in response to a prescribed pulse and a comparator C comparing a voltage of an input signal outputted from the sample-and-hold circuit with a prescribed reference voltage are provided. Thus, a reference voltage source PB supplying a prescribed reference voltage at any time based on the conversion data up to the pre-stage to the comparator C in existence to each bit is provided. Thus, before the processing at the final stage is finished, the succeeding (digital conversion) analog signal is inputted sequentially to an A/D converter 10 to attain the processing required for each stage. Thus, the A/D conversion is applied without increasing number of components.