  Gravitational-wave observation together with a large number of
electromagnetic observations shows that the source of the latest
gravitational-wave event, GW170817, detected primarily by advanced LIGO, is the
merger of a binary neutron star. We attempt to interpret this observational
event based on our results of numerical-relativity simulations performed so far
paying particular attention to the optical and infra-red observations. We
finally reach a conclusion that this event is described consistently by the
presence of a long-lived hypermassive or supramassive neutron star as the
merger remnant, because (i) significant contamination by lanthanide elements
along our line of sight to this source can be avoided by the strong neutrino
irradiation from it and (ii) it could play a crucial role to produce an ejecta
component of appreciable mass with fast motion in the post-merger phase. We
also point out that (I) the neutron-star equation of state has to be
sufficiently stiff (i.e., the maximum mass of cold spherical neutron stars,
M_max, has to be appreciably higher than 2M_sun in order that a long-lived
massive neutron star can be formed as the merger remnant for the binary systems
of GW170817, for which the initial total mass is >~ 2.73M_sun and (II) no
detection of relativistic optical counterpart suggests a not-extremely high
value of M_max approximately as 2.15-2.25M_sun.
