Abstract: A wrist watch to which an engagement structure of the present invention is applied includes a recessed body connected to a band, and a watch having a projected body formed on the rear side of a display portion. The projected body has an outer perimeter wall portion capable of fitting in an inner perimeter wall portion of the recessed body. The inner perimeter wall portion has a wall surface formed by joining circles with their centers at a second reference axis and with their diameters larger as the centers are shifted to the side of the watch portion. The outer perimeter wall portion has a wall surface formed by joining circles with their centers at a first reference axis and with their diameters smaller as the centers are shifted to the side of the recessed body. A recessed portion provided at the outer perimeter wall portion is engaged with a projected portion provided at the inner perimeter wall portion, and the watch and the recessed body are disengaged by relative rotation around the reference axis.