| 摘要 |
A center electrode of a spark plug has an oval shape in cross section as an identification mark. An electrode positioning apparatus fixedly holds a ground electrode and rotates an insulator together with a center electrode relative to the ground electrode. During the rotation, a signal light is irradiated toward the center electrode and the shape of the identification mark is detected from the received signal light. When the shape of the mark comes to correspond to a predetermined pattern, the center electrode is stopped from rotating and fitted solidly with the ground electrode.
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