We have successfully observed the ESR signals of radical cations in the thin films of N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (NPB), typical hole-transport material, for the first time to our best knowledge. In order to characterize the radical cation state, ESR spectra obtained upon chemical doping with iodine are analyzed combined with density functional theory (DFT) calculation. The ESR line width is inversely correlated with doping concentration.