  Recent observations have successfully detected UV-bright and infrared-bright
galaxies in the epoch of reionization. However, the origin of their radiative
properties has not been understood yet. Combining cosmological hydrodynamic
simulations and radiative transfer calculations, we present predictions of
multi-wavelength radiative properties of the first galaxies at $z\sim 6-15$.
Using zoom-in initial conditions, we investigate three massive galaxies and
their satellites in different environment and halo masses: $M_{\rm h} =
2.4\times 10^{10}{\rm M_{\odot}}$ (Halo-10), $1.6\times 10^{11}{\rm M_{\odot}}$
(Halo-11) and $0.7\times 10^{12}{\rm M_{\odot}}$ (Halo-12) at $z=6$. We find
that most of gas and dust are ejected from star-forming regions by supernova
feedback, which allows UV photons to escape. We show that the peak of the
spectral energy distribution (SED) rapidly changes between UV and infrared
wavelengths on a time-scale of $\sim$\,100\,Myrs due to intermittent star
formation and feedback, and the escape fraction of UV photons fluctuates in the
range of $0.2-0.8$ at $z<10$ with a time-averaged value of 0.3. When dusty gas
covers the star-forming regions, the galaxies become bright in the
observed-frame sub-millimeter wavelengths. We predict the detectability of
high-$z$ galaxies with the Atacama Large Millimeter Array (ALMA). For a
sensitivity limit of $0.1\,{\rm mJy}$ at $850\,{\rm \mu m}$, the detection
probability of galaxies in halos $M_{\rm h} \gtrsim 10^{11}\,{\rm M_{\odot}}$
at $z\lesssim 7$ exceeds fifty per cent. We argue that supernova feedback can
produce the observed diversity of SEDs for high-$z$ galaxies.
