A casing in which an electric connector is housed, including a floor, and a wall standing perpendicular to the floor, the wall being formed with an opening, when a part of the electric connector projects outwardly through the opening, a seal formed around the part making close contact with an inner edge of the opening, the casing further including a first positioning unit for positioning the electric connector relative to the opening in a first direction defined as a length-wise direction of the wall, and a second positioning unit for positioning the electric connector relative to the opening in a height-wise direction of the wall.