Image processing is rapidly becoming popular in applied geology. However, it is often difficult to extract the object from other parts by image processing. In order to facilitate visual recognition, pre-processing before image input is useful. Therefore, the object is coated with the fluorescent paint on the photograph, and then examined under ultraviolet ray. The technique is effective for image processing because obtained images display a great difference in both brightness and color between objects and other parts. Three images were obtained by a CCD camera using optical filters (red, green and blue filter), and processed in several steps, including image sharpening, thresholding, and inter-picture operation.The gold vein in the Hishikari Mine were examined through the method described above. The gold veins in the mine were classified as three types (network veins, a banded vein, and complex veins with the different vein widths from the geological point of view). They demonstrate that all types of veins can be extracted practically from the host rock. The ore grade can accurately and easily be calculated by image analysis combined with chemical analysis.