A sensor of the present invention includes a sensor element; a metallic shell; an outer tube member fixed to a rear end portion of the metallic shell; and a protective outer tube member covering an outer surface of the outer tube member. The outer tube member has a seal portion being circumferentially in contact with the protective outer tube member directly or via another member, and a front-side separation portion located frontward of the seal portion and spaced apart from and facing the protective outer tube member. A metal coating layer (plating layer) is formed on the outer surface of at least the front-side separation portion of the outer tube member.