  We investigate the high-energy neutrino emission expected from newly born
magnetars surrounded by their stellar ejecta. Protons might be accelerated up
to 0.1-100 EeV energies possibly by, e.g., the wave dissipation in the winds,
leading to hadronic interactions in the stellar ejecta. The resulting PeV-EeV
neutrinos can be detected by IceCube/KM3Net with a typical peak time scale of a
few days after the birth of magnetars, making the characteristic soft-hard-soft
behavior. Detections would be important as a clue to the formation mechanism of
magnetars, although there are ambiguities coming from uncertainties of several
parameters such as velocity of the ejecta. Non-detections would also lead to
useful constraints on the scenario.
