A control apparatus that controls a gas concentration sensor includes a sweep circuit, a current detection resistor, and a calculation portion. The sweep circuit supplies the gas concentration sensor with a sweep current. The calculation portion calculates impedance of the gas concentration sensor. The gas concentration sensor and the current detection resistor are sequentially connected in series along a direction from the sweep circuit to a reference voltage. The sweep circuit has a constant voltage circuit and a reference resistor. An increasing and decreasing tendency of a manufacturing variation of the reference resistor and an increasing and decreasing tendency of a manufacturing variation of the current detection resistor are identical. The calculation portion divides a product of a resistance value of the current detection resistor and a time variation of applied voltage to the gas concentration sensor by a time variation of applied voltage to the current detection resistor.