  We investigate the potential use of nebular emission lines in the rest-frame
far-infrared (FIR) for determining spectroscopic redshift of z>8 galaxies with
the Atacama Large Millimeter/sub-millimeter Array (ALMA). After making a line
emissivity model as a function of metallicity, especially for the [O III] 88
micron line which is likely to be the strongest FIR line from H II regions, we
predict the line fluxes from high-z galaxies based on a cosmological
hydrodynamics simulation of galaxy formation. Since the metallicity of galaxies
reaches at ~0.2 Zsun even at z>8 in our simulation, we expect the [O III] 88
micron line as strong as 1.3 mJy for 27 AB objects, which is detectable at a
high significance by <1 hour integration with ALMA. Therefore, the [O III] 88
micron line would be the best tool to confirm the spectroscopic redshifts
beyond z=8.
