A connector comprising: a terminal fitting comprising a body portion having substantially square tube shape; and a housing comprising a cavity that receives the terminal fitting when the terminal fitting is inserted into the cavity, wherein the body portion comprises a first protrusion that protrudes from an outer surface of the body portion and an interference avoiding portion, wherein the body portion is formed by a plurality of wall portions, wherein a boundary portion between adjacent two of the wall portions that are adjacent to each other substantially at right angles is connected substantially as a quarter arc shape to form the interference avoiding portion, and wherein the first protrusion and the interference avoiding portion are disposed to be opposed to each other substantially in a diagonal line on a virtual projection plane where the first protrusion and the interference avoiding portion are projected in a direction in which the terminal fitting is inserted into the cavity, characterized in that the cavity is formed with, in an inner surface thereof, a first groove into which the first protrusion is allowed to enter only when the terminal fitting has a proper insertion posture relative to the cavity and an interference portion that is formed to swell from the inner surface of the cavity to narrow a gap with the interference avoiding portion such that when the terminal fitting is intended to be inserted in an improper posture in which the terminal fitting is inverted around a symmetric axis parallel to an insertion direction, the first protrusion is locked in abutment against an area of an opening edge portion of the cavity located on the opposite side to the cavity with interposition of the interference portion. a terminal fitting comprising a body portion having substantially square tube shape; and a housing comprising a cavity that receives the terminal fitting when the terminal fitting is inserted into the cavity, wherein the body portion comprises a first protrusion that protrudes from an outer surface of the body portion and an interference avoiding portion, wherein the body portion is formed by a plurality of wall portions, wherein a boundary portion between adjacent two of the wall portions that are adjacent to each other substantially at right angles is connected substantially as a quarter arc shape to form the interference avoiding portion, and wherein the first protrusion and the interference avoiding portion are disposed to be opposed to each other substantially in a diagonal line on a virtual projection plane where the first protrusion and the interference avoiding portion are projected in a direction in which the terminal fitting is inserted into the cavity, characterized in that the cavity is formed with, in an inner surface thereof, a first groove into which the first protrusion is allowed to enter only when the terminal fitting has a proper insertion posture relative to the cavity and an interference portion that is formed to swell from the inner surface of the cavity to narrow a gap with the interference avoiding portion such that when the terminal fitting is intended to be inserted in an improper posture in which the terminal fitting is inverted around a symmetric axis parallel to an insertion direction, the first protrusion is locked in abutment against an area of an opening edge portion of the cavity located on the opposite side to the cavity with interposition of the interference portion.