  We perform long-term general relativistic neutrino radiation hydrodynamics
simulations (in axisymmetry) for a massive neutron star (MNS) surrounded by a
torus, which is a canonical remnant formed after the binary neutron star
merger. We take into account the effects of viscosity, which is likely to arise
in the merger remnant due to magnetohydrodynamical turbulence. As the initial
condition, we employ the azimuthally averaged data of the MNS-torus system
derived in a three-dimensional, numerical-relativity simulation for the binary
neutron star merger. The viscous effect plays key roles for the remnant
evolution and mass ejection from it in two phases of the evolution. In the
first $t\lesssim10$ ms, a differential rotation state of the MNS is changed to
a rigidly rotating state, and as a result, a sound wave, which subsequently
becomes a shock wave, is formed in the vicinity of the MNS due to the variation
of the quasi-equilibrium state of the MNS. The shock wave induces significant
mass ejection of mass $\sim(0.5-2.0)\times 10^{-2}M_\odot$ for the alpha
viscosity parameter of $0.01-0.04$. For the longer-term evolution with $\sim
0.1-10$ s, a significant fraction of the torus material is ejected. The ejecta
mass is likely to be of order $10^{-2}M_\odot$, so that the total mass of the
viscosity-driven ejecta could dominate that of the dynamical ejecta of mass
$\lesssim 10^{-2}M_\odot$. The electron fraction, $Y_e$, of the ejecta is
always high enough ($Y_e\gtrsim0.25$) that this post-merger ejecta is
lanthanide-poor; hence, the opacity of the ejecta is likely to be $\sim 10-100$
times lower than that of the dynamical ejecta. This indicates that the
electromagnetic signal from the ejecta would be rapidly evolving, bright, and
blue if it is observed from a small viewing angle ($\lesssim 45^\circ$) for
which the effect of the dynamical ejecta is minor.
