Based on the results for temperature dependences of optical birefringence and optical indicatrix rotation derived for Sn2P2S6 crystals, we show that at the atmospheric pressure, those crystals are under the conditions of tricritical point, with the critical exponent of the order parameter ? . At the same time, we have found that the temperature behaviour of the rotation angle of spontaneous electric polarization obeys the law ? . The action of hydrostatic pressure leads to only small changes in the critical exponent β, thus suggesting that the tricritical point on the P, T-diagram of Sn2P2S6 should occur in the pressure range ? .