A sampling circuit in a successive approximation type analog-to-digital (A/D) converting device samples a pair of analog signals constituting a differential input signal. A capacitor circuit reflects a signal level of a reference signal in the pair of analog signals through an attenuation capacitance unit and a binary capacitance unit to generate a pair of voltage signals. A comparison circuit compares the pair of voltage signals. A control circuit determines a value of each bit of a digital signal on the basis of the result of the comparison and reflects the value in the reference signal. The attenuation capacitance unit includes a fixed capacitance unit connected between a signal node at which the sampled analog signals are held and a predetermined potential node and a variable capacitance unit connected between the signal node and the predetermined potential node in parallel with the fixed capacitance unit.