To improve inspection accuracy of a spark gap by taking a picture of a spark gap so that the whole of the front end edge of an earth electrode facing the spark gap and only a part of an outer circumference edge of a central electrode including the whole part facing the spark gap are included inside a view field.
A camera takes a picture of a spark gap(g) of a workpiece at a predetermined magnification so that the whole of an electrode edge line E2 of an earth electrode W2 facing the spark gap(g) and only a part of an outer circumference edge of the front end face of a central electrode W1 including the whole of an electrode edge line E1 facing the spark gap(g) are included in a view field 210. In this case, the magnification is set so that a half or more of the circumference of the electrode edge line E1 of the front end face in the central electrode W1 is included inside the view field 210. For increasing the magnification more, less than the half circumference of the outer circumference edge E may be included in the view field 210. However, the whole of the electrode edge line El facing the spark gap(g) must be included.
NOZAWA KAZUHIRO
KOUMATSU SHINICHIRO
