Ultralow-lying magnon energy spectrum in yttrium iron garnet (YIG) has been studied by inelastic neutron scattering in an energy range from 10 to 45 mu eV. When a magnetic field of approximately 0.1 T was applied along [111] direction, ultralow-energy magnon anomaly was found at 10 K, suggesting the closure of the Zeeman energy gap. The anomaly was also observed in the temperature dependence of magnetization under a magnetic field along the [111] direction below 30 K. The specific heat capacity confirms the closure of the Zeeman energy gap at the magnetic field direction. All these anomalies strongly support the magnetic crossover below 30 K under the magnetic field along [111], in addition to a precursor anomaly below 150 K.