An impedance detector includes a current application portion, a detection portion, a calculation portion and a changing portion. The current application portion applies an impedance detection current for detecting an impedance of an element of an oxygen sensor to the element. The detection portion detects a difference between a current application prior voltage and a current application subsequent voltage of the element. The calculation portion calculates the impedance of the element based on the difference detected by the detection portion and a value of the impedance detection current applied by the current application portion. The current application portion can change the value of the impedance detection current applied to the element. The changing portion determines to change the value of the impedance detection current according to the difference detected by the detection portion.