  Recent observations have successfully detected UV or infrared flux from
galaxies at the epoch of reionization. However, the origin of their radiative
properties has not been fully understood yet. Combining cosmological
hydrodynamic simulations and radiative transfer calculations, we present
theoretical predictions of multi-wavelength radiative properties of the first
galaxies at z=6-15. We find that most of the gas and dust are ejected from
star-forming regions due to supernova (SN) feedback, which allows UV photons to
escape. We show that the peak of SED rapidly shifts between UV and infrared
wavelengths on a timescale of 100 Myr due to intermittent star formation and
feedback. When dusty gas covers the star-forming regions, the galaxies become
bright in the observed-frame sub-millimeter wavelengths. In addition, we find
that the escape fraction of ionizing photons also changes between 1-40% at
z>10. The mass fraction of HII region changes with the star formation history,
resulting in the fluctuations of metal lines and Lyman-alpha line luminosities.
In the starbursting phase of galaxies with the halo mass $\sim 10^{11}\,{\rm
M_{\odot}}$ ($10^{12}\,{\rm M_{\odot}}$), the simulated galaxy has $L_{\rm
[O\,III]} \sim 10^{42}\, (10^{43})\, {\rm erg\,s^{-1}}$, which is consistent
with the observed star-forming galaxies at z>7. Our simulations suggest that
deep [C II] observation with ALMA can trace the distribution of neutral gas
extending over $\sim 20$ physical kpc. We also find that the luminosity ratio
$L_{\rm [O\,III]}/L_{\rm [C\,II]}$ decreases with bolometric luminosity due to
metal enrichment. Our simulations show that the combination of multi-wavelength
observations by ALMA and JWST will be able to reveal the multi-phase ISM
structure and the transition from starbursting to outflowing phases of high-z
galaxies.
