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 lambda-line in a (2L, T) phase diagram, where T is the temperature and 2L is the cell thickness, is determined numerically. The critical temperature T-c of the SmA <-> SmC* (SmC) phase transition exibits nonmonotonic dependence on the cell thickness 2L. Two tricritical points are found on the lambda-line, which could be candidates for the Lifshitz points.