  We have performed very large and high resolution cosmological hydrodynamic
simulations in order to investigate detectability of nebular lines in the
rest-frame ultraviolet (UV) to optical wavelength range from galaxies at $z >
7$. We find that the expected line fluxes are very well correlated with
apparent UV magnitudes. The C IV $1549 \rm \AA$ and the C III] $1909 \rm \AA$
lines of galaxies brighter than $26~ \rm AB$ are detectable with current
facilities such as the VLT/XShooter and the Keck/MOSFIRE. Metal lines such as C
IV $1549 \rm \AA$, C III] $1909 \rm \AA$, [O II] $3727 \rm \AA$ and [O III]
$4959/5007 \rm \AA$ can be good targets for the spectroscopic observation with
Thirty Meter Telescope (TMT), European Extremely Large Telescope (E-ELT), Giant
Magellan Telescope (GMT) and James Webb Space Telescope (JWST). H$\alpha$ and
H$\beta$ lines are also expected to be detectable with these telescopes. We
also predict detectability of the nebular lines for $z > 10$ galaxies that will
be found with JWST, Wide-Field Infrared Survey Telescope (WFIRST) and First
Light And Reionization Explorer (FLARE) $(11 \leq z \leq 15$). We conclude that
the C IV $1549 \rm \AA$, C III] $1909 \rm \AA$, [O III] $4959/5007 \rm \AA$ and
H$\beta$ lines even from $z \sim 15$ galaxies can be strong targets for TMT,
ELT and JWST. We also find that the magnification by gravitational lensing is
of great help to detect such high-$z$ galaxies. According to our model, the C
III] $1909 \rm \AA$ line in $z > 9$ galaxy candidates is detectable even using
the current facilities.
