Ellipsometric studies have been conducted on freely-suspended films of an antiferroelectric liquid crystal mixture showing V-shaped switching. We observed a discontinuous change of the phase difference between s- and p- components of transmitted light Delta, at 46 degrees C indicating a phase transition from smectic-C* to the ferrielectric phase, although this transition is continuous in conoscopic observation. We also discussed the details of Delta as a function of an applied electric field in each phase.