A plate member joining structure includes a lower plate that is made of metal, a fitting base portion being formed at an upper surface of the lower plate; an upper plate that is made of metal, that is disposed on the upper surface of the lower plate, and that has a fitting insert portion that is fitted to the fitting base portion so as to form a first gap between a lower surface of the fitting insert portion and a surface of the fitting base portion facing the lower surface of the fitting insert portion; and a screw that, in a state in which the fitting insert portion is fitted to the fitting base portion, is screwed into the fitting insert portion and the fitting base portion, and joins the fitting insert portion and the fitting base portion together in a plate thickness direction.