The optical properties of ferromagnetic FeCoSiB alloy in the amorphous state and after temperature treatments have been investigated by spectroscopic ellipsometry in the energy range of 0.073-5.64 eV. It is shown that alloy crystallization changes significantly the frequency dependences of the optical constants and the dielectric functions calculated based on them. The atomic ordering is accompanied by a significant increase in the intensity of the fundamental band of quantum light absorption. The plasma and relaxation frequencies of conduction electrons have been calculated.