METHOD OF MANUFACTURING SPARK PLUG

A method for manufacturing a spark plug includes: a preparation step of preparing a specimen provided with a center electrode, an insulator, a metal shell, and a ground electrode; and an inspection step of applying a predetermined voltage between the ground electrode and the center electrode of the prepared specimen, and determining whether dielectric breakdown is caused in the insulator. The inspection step includes a calculation step and a judgment step. The calculation step involves receiving an oscillating wave generated from the specimen upon application of the predetermined voltage to the specimen, obtaining a power spectrum by subjecting an oscillating wave signal representing the oscillating wave to fast Fourier transform, and then calculating an integral of a predetermined frequency range in the power spectrum. In the judgment step, whether discharge is caused by dielectric breakdown of the insulator is judged by utilizing the integral.