Optical transmission and attenuated total reflection (ATR) measurements have been performed on antiferroelectric MHPOBC(R) liquid crystal films in the chiral smectic C-A phase 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.