IceCube has reported detection of three events of neutrinos with energies around 1 PeV and addi-
tional events at lower energies, which significantly deviate from the expected level of background
events. It is necessary to observe GeV-TeV photon, EeV-ZeV hadron and TeV-PeV neutrino all
together, in order to understand hadronic interactions of cosmic rays in the PeV-EeV energy re-
gion. It is required to make a step further toward exploring the PeV-EeV universe with high
accuracy and high statistics observations for both neutrinos and gamma rays simultaneously, by
using the instrument such as Ashra Neutrino Telescope Array (NTA). Wide and fine survey of
gamma-rays and neutrinos with simultaneously detecting Cherenkov and fluorescence light with
NTA will guide us to a new intriguing stage of recognizing astronomical objects and non-thermal
phenomena in ultra-high energy region, in addition, new aspect about the fundamental concepts
of physics beyond our presently limited understanding; the longstanding problem of cosmic ray
origin, the radiation mechanism of gamma-rays, neutrino and cosmic rays from violent objects
like blazars, interaction of gamma-rays and cosmic rays with microwave and infrared background
photons, and PeV-EeV neutrinos originated from far places beyond the GZK-horizon.