<i>Shakudo</i> is a copper alloy containing 1-5 wt% gold. Although * 2shakudo*1 is famous for its black surface, the black surface does not appear before the alloy is treated by <i>nikomi-chakushoku</i>, a traditional Japanese coloring method, which means literally coloring by boiling. The <i>shakudo</i> specimens were experimentally prepared. X-ray diffraction of the specimens revealed that the colored layer of the experimental <i>shakudo</i> consisted mainly of cuprite. <i>Shakudo</i> used for actual artifacts was also examined by nondestructive x-ray diffraction analysis. It was confirmed that the colored layer of the historical <i>shakudo</i> also consisted mainly of cuprite. -- AATA