Optical transmission and attenuated total reflection (ATR) measurements have been performed on antiferroelectric MHPOBC(R) liquid crystal films in the chiral smectic CAphase to investigate the electric field response of the bulk and boundary layers, respectively. In the ATR measurement, a surface plasmon polariton is excited at the liquid-crystal/silver-film interface as a probe of the field response of the boundary layer. A significant difference in threshold voltage as well as in field responsivity is found between the bulk and boundary layers. These results are discussed in short in this letter.