A guide protrusion includes first walls protruding upwards from an upper face of a body, facing each other across an opening in a first direction, and first protrusions protruding from each first wall toward the other. A receptive portion between the first walls has a first opening, opening to one side in a second direction orthogonal to the first direction. The sensor case is received from the first opening following the second direction. The guide protrusion has a first movement restricting face facing another side in the second direction. The sensor case includes a columnar portion extending vertically through a gap between the first protrusions, a flange at least partially between the upper face and the first protrusions in the vertical direction, within the receptive portion, and a facing portion on the other side of the first movement restricting face in the second direction, facing the first movement restricting face.