A gas sensor which detects a concentration of a specific gas contained in a gas to be measured is provided with a gas sensor element. The gas sensor element has an ion conductive solid electrolyte body, a measuring gas electrode which is mounted on a surface of the solid electrolyte body, a reference gas electrode mounted on a surface of the solid electrolyte body, and a catalyst layer mounted on an outer-side relative to the measuring gas electrode-side. The catalyst layer contains a metal catalyst loaded onto a carrier. The metal catalyst is a Pt single substance, and has a specific surface area defined by the equation below of equal to or more than 0.01 to equal to or less than 23: Specific surface area=total surface area of metal catalyst which exists on an electrode unit surface area/actual surface area of electrode per electrode unit surface area.