When an operated period of a structure is close to or over the original design life, it is necessary to reassess the regular inspection program applied to secure the structural integrity. Because, the program is effective only to detect damages in the structural details which occur within the design life. Therefore, another inspection program, that is, a supplemental inspection program which is intended to detect significant damages occurring after the design life with high detection capability, is required for the sake of continuing safe operation of ageing structure. As to aircraft structures, FAA and ICAO have recently imposed to develop a supplemental inspection document of older aircrafts on manufacturers. In response to the requirements, the Boeing company has released the supplemental inspection documents for Model 707/720 and 727. The purposes of the present study are ; 1) to define deterministic and probabilistic factors which are necessary in a reliability analysis on the effects of supplemental inspection, 2) to examine results obtained by the Boeing method, and 3) to propose a method to estimate possible extension of service life resulting from a supplemental inspection. A numerical example of typical aircraft structural significant detail, which is a wing spanwise skin splice consisting of skin and stringer, is given in order to evaluate the validity of the present study.