PURPOSE: To easily detect a flaw such as a crack generated in a structural element by providing an optical fiber for damage detection inside or on the surface of the structural element.

CONSTITUTION: Plural optical fibers 3 are laid at equal intervals on the surface of the external wall member 1 of an aircraft formed of prepreg. Their projection-side end parts 5 are bundled at one position, end surfaces are arranged in order to facilitate light incidence, and photodetection-side end parts 6 are projected from an end surface of the external wall member 1. There is no light leak in each optical fiber 3 unless there is a damage between both ends 5 and 6. Those optical fibers 3 laid as mentioned above are cut by a flaw A in the structural element 1 along with the main body of the structural element 1, so light incident from one end 5 of optical fibers 3 leak at the cracking part on the side of a light source B, so the state is easily confirmed at the cracking part or the other end 6.