By utilizing the sharpness of a focused ion beam, lateral resolution of SIMS analysis has been improved by one or two orders of magnitude. The authors constructed the submicron SIMS by using a gallium focused ion beam. On the other hand, a focused ion beam is frequently used as a tool for micromachining due to its high current density and sharpness.Insitucombination of micromachining and high spatial resolution analysis brings a new field of microbeam analysis on materials. In the development of thin‐film multilayer devices or large scale integrated circuits, three‐dimensional analysis of local microstructure has a great importance. In order to determine location and composition of a specific subsurface microstructure, a focused ion beam is used as (1) a machining tool to expose a vertical cross section on which the center of the specific microstructure lays, and (2) a primary beam of SIMS element mapping. Through the analysis of a buried fine particle, the submicron SIMS is shown to be a powerful tool for three‐dimensional analysis of a microstructure.