  We perform a general-relativistic magnetohydrodynamics simulation for
$\approx 30$ ms after merger of a binary neutron star to a remnant massive
neutron star (RMNS) with a high spatial resolution of the finest grid
resolution $12.5$ m. First, we estimate that the Kelvin-Helmholtz instability
at merger could amplify the magnetic-field energy up to $\sim 1\%$ of the
thermal energy. Second, we find that the magnetorotational instability in the
RMNS envelope and torus with $\rho < 10^{13}~{\rm g~cm^{-3}}$ sustains
magneto-turbulent state and the effective viscous parameter in these regions is
likely to converge to $\approx 0.01$--$0.02$ with respect to the grid
resolution. Third, the current grid resolution is not still fine enough to
sustain magneto-turbulent state in the RMNS with $\rho \ge 10^{13}~{\rm
g~cm^{-3}}$.
