PURPOSE: To decrease the number of component elements and to facilitate the formation of an IC, by comparaing successively the voltage dividing level between reference voltage levels with an analog input, through a multiplexer basing on a binary researching system.

CONSTITUTION: When an A-D conversion instruction STC is applied to a sequencer 11, the voltage level of an analog input AIN is compared with each voltage dividing level. The result of this comparison is stored in a 3-bit register 2. The state is decided for multiplexers 3 and 4 from the contents of 3-bit digital data D0∼D2, and reference voltages VR1 and VR2 are produced by amplifiers 5 and 6. Then the voltage dividing level of resistance between the VR1 and VR2 is compared with the input AIN by a binary researching system comprising a multiplexer 7, a comparator 8 and a sequential comparison register 9. Thus the digital data of lower 5 bits is obtained. Then a conversion end signal EOC is delivered when the final data D7 is obtained. Thus the digital data D1∼D7 are all obtained to the input AIN.