We present the ALMA Band 6 (1.3 mm and 1,1 mm) and ACA Band 7 (0.83 mm) observations of the starless core G208.68-19.02-N2 (G208-N2) in the Orion Molecular Cloud 3 (OMC-3). The dust continuum emission shows that this core has a filamentary structure with a length of ~5000 au and a H2 volume density of ~6 x 10^7 cm^-3. At the tip of this filamentary structure, there is a compact "nucleus" with a radius of ~200 au and a mass of ~0.1 M_sun. The nucleus is extremely dense: it has a central density of ~2 x 10^9 cm^-3 with a radial density profile of r^-1.4 . The nucleus does not have a counterpart in the wavelength shorter than 70 micron. In addition, there is no signature of outflow localized to the nucleus in the CO 2-1 image. The C18O 2-1 emission is completely missed in the filamentary structure including the nucleus. The N2D+ 3-2 emission traces the filamentary structure. These imply the significant CO depletion due to high density and cold temperature in the filament and nucleus. Interestingly, the N2D+ emission
does not peak toward the nucleus. Furthermore, the ortho-H2D+ emission at 372 GHz does not peak toward the nucleus.The depletion of N2D+ and H2D+ in the nucleus could imply the possible depletion of HD, a parent molecule of H2D+. If it is the case, the nucleus is still in the prestellar phase before the onset of the first hydrostatic core formation. We also found that the very narrow (~0.2 km/s) DCO+ 5-4 emission component peaks toward the nucleus. This could imply that the small amount of CO has been desorbed from the dust due to the formation of internal heating source such as a first hydrostatic core.