Abstract: In an AD converting device in which an analog random noise is added by analog adding means to an analog signal to be converted and the added output is converted by an AD converter to a digital signal, an analog offset voltage, whose level variation is around (1 to 7) times the quantization step size .DELTA. of the AD converter, is generated by offset generating means and is also applied to the analog adding means. A random noise waveform is stored in a memory, the read-out output of which is converted by DA converting means to an analog random noise having an amplitude about (1/2 to 6).DELTA., and the converted output is used as the above-said random noise. A digital value of the offset voltage and the digital noise waveform are subtracted from the output of the AD converter, as required.