A thin free-standing film of an antiferroelectric liquid crystal subphase was studied by means of the precise measurement of transmission ellipsometry. At a temperature in which the SmA phase is observed in the bulk sample, a small director tilt and helical structure was confirmed from the optical phase difference measurement, and an unwinding process under a high electric field was observed. The dependence of the threshold characteristics on layer numbers was also recognized. From the experimental results, it is estimated that the structure of SmCalpha* is a tilted smectic structure with a small tilt angle and a fairly short pitch.