PURPOSE: To enable detection of a burying of the tip of a lead in a solder by an optical means by utilizing a light reflection characteristic of a soldered part and the lead.

CONSTITUTION: An optical scanning is performed widthwise at a base end part of a lead l to set a point (a) equally dividing a lead width. Then, with the point (a) as starting position of the optical scanning, the optical scanning is performed by a specified distance D along the length of the lead L. Then, when a light presentation continues in the brightness, with no change from a light to dark level, the tip of the lead L is determined to be buried in a soldered part 9. On the other hand, a filet 10 is observed dark with a camera 3 as light incident from above is reflected sideways while the brightness is changed clearly to a dark level thereby enabling the confirming of the presence of the filet 10. This simplifies the detection of whether the tip of the lead is buried in a solder or not.