To perform high-accuracy A/D conversion while remarkably reducing the number of external terminals to be used for an A/D converter.

During sampling, switches 21-25 are turned on and a switch 26 is turned off. Since a resistor 19 is set to an optimal resistance value during sampling, impedance seen from a node A to the left side is approximately matched with impedance seen from a node B to the left side, and great noise cancel effects can be obtained. During sequential comparison, switches 21-23, 25 are turned off and switches 24, 26 are turned on. Since a resistor 20 is set to an optimal resistance value during sequential comparison, impedance seen from the node A to the left side is approximately matched with impedance seen from the node B to the left side, and great noise cancel effects can also be obtained during sequential comparison.