Non-destructive diagnosis using an optical fiber sensor was developed to evaluate the aging degree of thermally deteriorated insulating resins. The key point of this technique is measuring the change of optical losses in an insulating resin at the two different near-infrared wavelengths. The sensor consists of a plastic optical fiber, a plastic optical coupler, two kinds of LED light sources, and an optical power meter, so it is very compact and light (less than 2kg). The aging degree of a material could be estimated using a diagnostic master curve, which is obtained by a accelerated heating test. First, this diagnosis is applied to the prediction of the residual life of elevator motors.