An infrared sensor includes an infrared detecting device, a lens, a member, a gap and a spacer. The lens is disposed above the infrared detecting device. The member forms an external surface and includes a first opening having a maximum internal diameter. The gap is disposed between the member and the lens. The spacer is disposed between the member and the lens so as to form the gap, and that is directly contact with lens. The spacer has a circular inner periphery, in planar view, which has a larger internal diameter than the maximum internal diameter of the first opening of the member.