Production method of spark plug

A spark plug production method of producing a spark plug which comprises an insulating member, a center electrode including a portion disposed at a forward end of the insulating member, a tubular outer metal member surrounding the insulating member, and a ground electrode connected with the outer metal member and arranged to confront the center electrode to form a discharge gap, the spark plug production method comprising:

• producing standard image data representing a standard image by capturing an image of a standard device including a slit forming section forming a slit which includes a standard slit segment having a predetermined width, with an image capturing device, the standard image being an image including an image of the slit of the standard device;
• determining a width of the standard slit segment of the slit in the standard image by analyzing the standard image data;
• producing gap image data representing a gap image which is an image including an image of the discharge gap, by capturing an image of the spark plug with the image capturing device;
• determining a distance of the discharge gap in the gap image, by analyzing the gap image data;
• and
• determining whether an actual distance of the discharge gap is within a predetermined allowable range or not, by using the distance of the discharge gap determined in the gap image and the width of the standard slit segment determined in the standard image.