The optical absorption edge on the indium-doped (N_In <= 0,8 weight. %) Pb_1-xSn_xSe (x = 0,07) epitaxial layers have been investigated. The observered movement of intrinsic absorption edge in the short-wave length region spectrum is interpretated by the presence of indium impurity, which lead to some increading of gap with E_g in Pb_1-xSn_xSe. It has been found, that the inter-zone absorption edge in the weak absorption region Pb_1-xSn_xSe is conditioned by nondirect optic transitions. The magnitudes of E_g and dE_g/dT have been calculated and are described by (K)^½=f(E) curve.