A protector includes an inside protector having an inside peripheral wall and a front end wall in a front end side thereof and a tubular outside protector which surrounds the inside protector. In an outside peripheral wall of the outside protector, a plurality of outside introducing ports through which an external part of the outside protector communicates with a gas separating chamber are formed at equal intervals along a circumferential direction. The outside introducing ports are formed at positions nearer to the front end side than positions where inside introducing ports of the inside protector are formed. The outside introducing ports extend in the circumferential direction of the outside peripheral wall and formed in shapes of lateral holes in which opening lengths in the circumferential direction are larger than opening lengths in the direction perpendicular to the circumferential direction.