Gas sensor

A protector has a two-stage structure that includes a large-diameter portion and a small-diameter portion. The large-diameter portion includes a cylindrical first peripheral wall and a first front end wall. The small-diameter portion includes a cylindrical second peripheral wall connected to the first front end wall and a second front end wall connected to a front end portion of the second peripheral wall. Opening portions are not formed in the first and second peripheral walls. First opening portions, which are opened toward only first recessed portions and an inner surface of the first peripheral wall, are formed at the first front end wall. A second recessed portion and second opening portions, which are formed in the second recessed portion and are opened toward an inner surface of the second peripheral wall, are formed in the second front end wall.