An NOx sensor includes a gas sensor element, a body member made of metal, and a protector made of metal. The body member is formed into a tubular shape extending in an axial direction and accommodates a gas sensor element internally of the same. The NOx sensor is formed into a tubular shape extending in the axial direction and includes an attachment member disposed such that a space extending in the axial direction is formed between the attachment member and the body member. The gas sensor element includes an oxygen concentration detection cell having an oxygen ion-conductive solid electrolyte layer and a detection electrode and a reference electrode formed on the solid electrolyte layer and forming a pair, and a heater for heating the oxygen concentration detection cell to a predetermined temperature. The oxygen concentration detection cell is disposed in the space.