A base plate includes a plate body and a connection portion provided at the plate body. The connection portion includes a through hole gassing through the plate body in a thickness direction of the plate body, a plurality of protrusions provided along an edge of the through hole in a circumferential direction about a central axis of the through hole and inserted into an attachment hole provided in the arm, and an inner circumferential surface formed by the plurality of protrusions and the through hole. The inner circumferential surface includes a plurality of recesses formed between the plurality of protrusions.