PURPOSE: To improve the accuracy for judging by detecting the area of a region where the minimum value of the distribution of the amount of X-ray transmission and the amount of X-ray transmission below a specific reference value are detected to judge whether soldering is proper or not based on the numeric values.

CONSTITUTION: When judging whether one longitudinal section of a lead part 10 of electronic parts 9 is proper or not, the distribution of the amount of X-ray transmission in the longitudinal direction of the lead part 10 of the electronic parts 9 is detected from an X-ray transmission image. A minimum value Pmin of the amount of X-ray transmission is obtained from the distribution of the amount of X-ray transmission. Then, by comparing a threshold Pdek used for judging the minimum value Pmin with the minimum value Pmin the amount of transmission which is larger than the threshold Pdek, an determined to be nonconforming. When the detected minimum value Pmin is the amount of transmission which is smaller than the threshold Pdek, an area Qarea of a region where the amount of X-ray transmission which is equal to or less than a preset reference value is detected is obtained. By comparing the detected area Qarea with the threshold Qdek which is judged to be conforming or not, a failure results when the detected area Qarea is smaller than the thresh Qdek.