  The merger of binary neutron stars (NSs) is among the most promising
gravitational wave (GW) sources. Next-generation GW detectors are expected to
detect signals from the NS merger within 200 Mpc. Detection of electromagnetic
wave (EM) counterpart is crucial to understand the nature of GW sources. Among
possible EM emission from the NS merger, emission powered by radioactive
r-process nuclei is one of the best targets for follow-up observations.
However, prediction so far does not take into account detailed r-process
element abundances in the ejecta. We perform radiative transfer simulations for
the NS merger ejecta including all the r-process elements from Ga to U for the
first time. We show that the opacity in the NS merger ejecta is about kappa =
10 cm^2 g^{-1}, which is higher than that of Fe-rich Type Ia supernova ejecta
by a factor of ~ 100. As a result, the emission is fainter and longer than
previously expected. The spectra are almost featureless due to the high
expansion velocity and bound-bound transitions of many different r-process
elements. We demonstrate that the emission is brighter for a higher mass ratio
of two NSs and a softer equation of states adopted in the merger simulations.
Because of the red color of the emission, follow-up observations in red optical
and near-infrared (NIR) wavelengths will be the most efficient. At 200 Mpc,
expected brightness of the emission is i = 22 - 25 AB mag, z = 21 - 23 AB mag,
and 21 - 24 AB mag in NIR JHK bands. Thus, observations with wide-field 4m- and
8m-class optical telescopes and wide-field NIR space telescopes are necessary.
We also argue that the emission powered by radioactive energy can be detected
in the afterglow of nearby short gamma-ray bursts.
