The phase transition from the SmA to the lower-temperature SmC* or SmC phase in a restricted geometry of a planar cell is studied theoretically, considering a polar coupling of the ferroelectric liquid crystal to the surfaces. The λ-line in a (2L, T) phase diagram, whereTis the temperature and 2Lis the cell thickness, is determined numerically. The critical temperatureTcof the SmA ⟷ SmC* (SmC) phase transition exibits nonmonotonic dependence on the cell thickness 2L. Two tricritical points are found on the λ-line, which could be candidates for the Lifshitz points.