We have developed a novel technique of the plane-view transmission electron microscopy (TEM) observation of silicon (Si) nanowires by covering them firmly with a holding layer of Si3N4. The use of the Si3N4 holding layer is shown to be effective to fix the fine structure of the sample without deformation during a focused ion beam (FIB) thinning process, to protect the sample from a high energy FIB, and to give high contrast in the TEM image of the crystalline fine structure. The Si3N4 holding layer is quite suitable, therefore, for plane-view TEM observation of the fine structure such as a Si nanowire. By means of the proposed observation technique, we have successfully observed clear features in the direction of length of narrow (16 nm) Si nanowires fabricated by using SiO2 sidewall masks and the extremely fine (3-5 nm) Si nanowires obtained by the successive self-limiting oxidation, Moreover, the plane-view TEM lattice image of the ultrafine Si nanowire was also observed. The plane-view TEM observation technique described here is useful to analyze various fine nanostructures and contributes to develop nanofabrication technology.