In order to determine the cause of gingival pigmentation appearing after restoration by full cast crown, the gingival tissues showing gingival pigmentation apparently due to the full cast crown were investigated histopathologically using light and electron microscopy. Additionally, elements in some of the samples were studied with an electron probe microanalyser. Under the light microscope, many dark brown depositions of varying sized were observed in a wide area of the gingival lamina propria. Reduction in the arrangement of the basal cells were observed in the epithelial layer in the neighborhood of these depositions. Examination under the electron microscope clarified that these depositions were accumulations of minute granules, that large depositions had been formed by the fusion of small ones, becoming depositions of high electron density, and that around these large depositions were fibroblasts injecting minute granules. Many minute granules were observed attached too bundles of collagen fibers. The full cast crowns contained Ag, Au, Pd, Cu and Zn, on the other hand, the depositions contained Ag, S, Ca, P and Si. The element of the minute granule was Ag, which was considered to be in the form of AgS Calcification was occurring in some of the large depositions.