The material α-RuCl₃ has attracted great attention as a promising candidate for the 2D Kitaev honeycomb model. Due to its proximity to the Kitaev quantum spin liquid, various spectroscopic works on α-RuCl₃ reveal both a magnetic continuum-like excitation suggestive of spin fractionalization and spin-wave-like modes from the antiferromagnetic order at low temperature. Despite numerous experimental and theoretical studies, magnetic excitations in the magnetically ordered state is not fully understood due to their complexity. In this talk, we provide a well describable picture of magnetic excitations in the magnetically ordered state of α-RuCl₃ by using inelastic neutron scattering (INS) measurement. At the low temperature below T_N (~6.5 K), the INS data shows sharp spin wave excitations distinguishable from the broad continuum. Our linear spin wave theory analysis identifies the spin wave modes and enable us to construct a minimal spin Hamiltonian of exchange interactions. By expanding spin wave analysis to higher order, we find that the spectral weight of continuum excitations is far beyond that of multi-magnon excitation. Furthermore, we present the field-dependence of magnetic excitation, and discuss about the possible field-induced quantum spin liquid state.