New frontiers of particle astrophysics have been opened by IceCube's discovery of high-energy cosmic neutrinos. Their origin is a new mystery in the field, and solving this problem may enable us not only to understand the physics of astrophysical sources but also to obtain important clues about old mystery, the origin of cosmic rays, and to utilize neutrinos as probes of neutrino properties, dark matter, and fundamental physics. In this talk, with emphasizing the relevance of multi-messenger approaches, I will review recent basic ideas on the origin of high-energy cosmic neutrinos, including the grand-unified model we recently proposed, and some implications for particle physics properties.
