The method to study phase transitions of thin systems on. the basis of a bulk phase diagram is applied to a freely suspended film of ferroelectric smectics showing a first-order phase transition in the bulk, where an ordering effect resulting from surface layers is replaced by an effective field that is conjugate to the order parameter. In the framework of a phenomenological free energy whose coefficients are determined form the experimental evidence about C7, the behavior of the transition is being clarified, which coincides with experimental finding, and especially the sinall shift, oftransition teinperature fi-onz the bulk one is elucidated. The mechanism. of the continuous change occurring in the system of thickness just below a critical thickness is clarified, where an unstable state that is never realized in the bulk appears at the interior layers. The behavior of this continuous change discloses the difference between the ordering effects resulting from the boundaries and the external fields.