Temperature dependencies of the characteristic optical paths difference D(Τ) of K2SeO4 crystal in the range of 80-295 K were measured using the Jamen type interferometer. Significant anomalies of D(Τ) dependencies in the vicinity of incommensurate-paraelectric phase transition (T, = 130 K) are observed. On the basis of D(Τ) temperature dependencies measured experimentally and linear expansion l(Τ) known, the temperature derivative of the refractive indices dn/dT are found to be negative in the ranges above and below the temperature T•, = 130 K.