  Binary neutron star merger (BNSM) offers an environment where fast
neutrino-flavor conversion (FFC) can vividly occur, that potentially leads to a
considerable change of neutrino radiation field. In this Letter, we investigate
global features of FFC by general relativistic quantum kinetic neutrino
transport simulations in spatial axisymmetry. Our result suggests that global
advection of neutrinos plays a crucial role in FFC dynamics. Although flavor
conversions occur ubiquitously in the early phase, they can be active only in a
narrow region in the late phase. This region includes an ELN-XLN Zero Surface
(EXZS), corresponding to a surface where electron-neutrinos lepton number (ELN)
equals to heavy-leptonic one (XLN). The EXZS is not stationary, but dynamically
evolve in a time scale of global advection. We also find that neutrinos can
undergo a flavor swap when they pass through the EXZS, resulting in
qualitatively different neutrino radiation fields between both sides of EXZS.
Our result suggests that EXZS is one of the key ingredients to characterize FFC
in BNSM.
