The sensor element, which can detect the concentration of a specified gas, is held with a housing with a front end thereof exposed. A protective cover includes an inner cover and an outer cover, and secured to the housing. A ratio φ1/φ2 is set to a range from 0.6 to 0.9, where φ1 represents an outer diameter of a portion where an inner gas aperture is formed in the inner cover, and φ2 represents an inner diameter of a portion where an outer gas aperture is formed in the outer cover.