To enhance the igniting performance of a spark plug, resistance against contamination and breakage, and the yield from the production line by confirming each spark gap formation, etc., simply and accurately in the process of installing an insulator in a metal body.

When an insulator 2 retaining a center electrode 3 is to be installed in a metal body 5 to constitute a single piece structure with an outside electrode 6, each spark gap 7 bounded by the outside electrode 6 and the foremost periphery of the center electrode 3 or insulator 2 is along with its dimension confirmed by an image processing device which incorporates a camera 10, and if the spark gap 7 is positioned outside the set range, the distance of each spark gap 7 is adjusted into the specified dimension by rotating or changing the insulator 2 appropriately, and thereby the correcting work for the spark gap thereafter is lessened and their heights are made even, which should lead to enhancement of the igniting performance and the resistance against contamination and breakage, and an optimum combination of insulator 2 and metal body 5 is obtained, so that it is possible to heighten the yield from the production line.