A field emission STEM has been developed, and applied to elemental analysis using a sector type energy loss spectrometer. An electron beam is focused to approximately 10Å diameter, and irradiates the specimen with a beam current of over 10−10A. Small particles are employed as specimens to investigate the capabilities of micro-area analysis. A boron nitride particle, about 200Å in size, reveals sharp K-shell excitation edges of boron and nitrogen in its energy loss spectrum. These signals are estimated to be produced from 7×l0−21 grams of boron and 9×10−21 grams of nitrogen. Iron and oxygen are detected from a single ferritin particle, whose core is approximately 60Å in size and composed of iron oxide hydrate molecules. The intensity ratio of the iron and oxygen signals is compared with the ratio of ionization cross-sections.