In a successive approximation AD converter, a noise generator outputs the output of a ΔΣ modulator as a noise signal. A selector circuit can output the noise signal, in place of a digital signal for generating a comparison-target voltage for the next bit, to a capacitor element of a capacitance DAC. During sampling of an analog input voltage, the noise signal is supplied to the capacitance DAC via the selector circuit, and thereafter normal successive approximation operation is executed.