Comparison of the observed evolution of the Ly-alpha transmitted flux in the spectra of four highest redshift quasars discovered by SLOAN survey with the theoretical prediction for this evolution based on the state-of-the-art numerical simulations of cosmological reionization already allows one to constrain the redshift of reionization to z _ { REI } = 6.2 \pm 0.1 { s } \pm 0.2 { r } , where systematic and random errors are given respectively .