Disclosed is a gas sensor having a metal shell, a holder placed in the metal shell and a sensor element inserted through an axial insertion hole of the holder. The holder has a recessed portion recessed toward the rear from a front end face of the holder. The sensor element has, at a front end part thereof, a detection portion covered with a porous protection layer such that a rear end of the porous protection layer is situated within the recessed portion of the holder and is located at the rear side with respect to the front end face of the holder while maintaining a space between an inner circumferential surface of the recessed portion and an outer circumferential surface of the porous protection layer.