A connector includes a protrusion. The protrusion protrudes toward a distal end side to serve as a distal end of the connector. The connector moves in a diagonal direction relative to a first separator. A worker positions the connector in a Z direction, before moving the connector. With an upward protrusion and an inward protrusion formed, the worker standing by a first side wall looks into a casing from above to see a portion around an attachment portion. When the connector is positioned close to the attachment portion, the worker looking into the casing from above cannot see a bottom surface of the connector. The worker can see the protrusion even when he or she cannot see the bottom surface.