The energy gap near Fermi surface due to the hybridization of Ce3+ 4f1 and conduction electrons is the key to understand the physical properties of the CeOs4Sb12 compound, such as Kondo insulating behavior, moment quench of Ce3+ at low temperatures and the characteristics of heavy fermion. In this work, inelastic neutron scattering spectra of powder CeOs4Sb12 sample were collected at different temperatures using LAM-D neutron spectrometer at KEK pulsed neutron source. The result indicates that the CeOs4Sb12 skutterudite is a Kondo insulator with a c-f interaction of 3.1 meV. The Debye temperature is determined as 317 K.