An approach for evaluating three-dimensional spatial distribution of precipitates buried within a bulk alloy by coherent x-ray diffraction microscopy (CXDM) is proposed. In this study, the outer shape and the internal structure of the micrometer-sized sample in a precipitation-hardened aluminum alloy are observed by CXDM. A high-electron-density region resulting from precipitates is clearly visualized, implying that the precipitates distribute inhomogeneously in the Al matrix at several tens of nanometer scale. This is the important result toward in situ observation of the formation of internal structures in metallic materials on the nanomesoscale by CXDM with x-ray free electron lasers.