The attenuated total reflection infrared (ATR-IR) spectra of trans-polyacetylene (trans-PA) film doped with sodium (n-doping) were observed in the range of 1900 to $700cm^{-1}$. The observed IR bands were attributed to negatively charged domains created by n-doping electrons. The doping-induced IR bands showed considerable difference from its pristine film. After doping, the out-of-plane CH deformation band of the strong $1010cm^{-1}$ region in the pristine film disappeared while several new bands were observed at 1600 (due to C=C stretching), 1400 (due to in-plane CH bending), 1290 and 1174 (due to CH stretching), and $880cm^{-1}$ (due to CC stretching) regions for Na-doped PA. In particular, a weak band of C=C stretching at $1600cm^{-1}$ was newly obtained for the first time in the present study. The electro conductivity of the doped trans-PA film was $10^2S/cm$ and the origins of doping-induced IR bands are discussed in terms of solitons and polarons.