An oxygen sensor includes a tubular casing; an oxygen detecting element installed in a leading end of the tubular casing; lead wires each extending from the oxygen detecting element to the outside of the tubular casing through an open base end of the tubular casing, each lead wire including a core wire of metal; a rubber bush plugged in the open base end of the tubular casing, the rubber bush having the lead wires passed therethrough; and a heat resisting structure for protecting the rubber bush from a high temperature. The heat resisting structure is constructed to satisfy the following inequality: 10%.ltoreq.Al/Ar.ltoreq.50%, wherein: Al: The sectional area in total of the lead wires; and Ar: The sectional area of the rubber bush.