We have developed a remaining service life diagnostic technology for insulators, which are important parts involved in determining the service life of power distribution equipment. Application of chemical evaluation and the Mahalanobis-Taguchi method enabled us to diagnose the insulators' degree of deterioration with great accuracy and nondestructively on-site. The theory, which derives the threshold value of surface resistivity for electric discharge initiation calculated from rated voltage, the form of an insulator, etc., was established and a remaining service life was presumed from the diagnostic results by the MT method and the year in which the master curve of service life and the threshold value intersect. It was clarified that a remaining service life could be presumed by this technology because there was a good relationship between the check examination results of electric discharge initiation in the laboratory and the presumption results.