A gas sensor having an axis including a detecting element; a metal shell; an outer cylinder; and a plug member, as defined herein, and the gas sensor includes a first protective member that covers an air vent of the plug member so as to protect the filter member, the protective member having a ventilation portion having an opening smaller than an opening of the air vent and at least partially overlapping the air vent in the axial direction, and at least one of the outer cylinder and the plug member is provided with a locking portion that limits rotation of the protective member around the axis.