A delamination development preventable structure is provided to prevent the development of delamination in a sandwich panel including a core and surface plates attached to the opposite surfaces of the core with respect to the thickness of the core.Arresters 5 are protruded inward with respect to directions parallel to the thickness of a core 3 from surface plates 4a and 4b. The development of a delaminated part formed by the separation of a part of the surface plate 4a from the core 3 and that of a delaminated part formed by the separation of a part of the surface plate 4b from the core 3 beyond the arresters 5 can be prevented. Thus delamination can be limited to a narrow local region and the spread of the delaminated part over the entire sandwich panel 2 can be prevented. Thus the development of delamination in the sandwich panel 2 can be prevented and the durability of the sandwich panel 2 can be enhanced.