  In 2017, LIGO-Virgo collaborations reported detection of the first neutron
star merger event, GW170817, which is accompanied by electromagnetic
counterparts from radio to gamma rays. Although high-energy neutrinos were not
detected from this event, mergers of neutron stars are expected to produce such
high-energy particles. Relativistic jets are launched when neutron stars merge.
If the jets contain protons, they can emit high-energy neutrinos through
photomeson production. In addition, neutron star mergers produce massive and
fast ejecta, which can be a source of Galactic high-energy cosmic rays above
the knee. We briefly review what we learned from the multi-messenger event,
GW170817, and discuss prospects for multi-messenger detections and hadronic
cosmic-ray production related to the neutron star mergers.
