The optical properties of Zn1-xMnxSe with x up to 0.78 grown by molecular beam epitaxy on GaAs (001) substrates are reported. The complex dielectric functions ε=ε1+iε2 are obtained by spectroscopic ellipsometry in the photon range from 3.5 to 5.5 eV at room temperature. The critical-point energies E1 and E1+Δ1 for x ≤ 0.52 are determined by fitting the calculated second-derivative spectra of ε to the standard line shape. For the sample with x = 0.78, the peak position in the ε2 spectrum is taken as an estimate for E1. While the fundamental band gap E0 increases with x, the E1 and E1+Δ1 energies decrease almost linearly with x. Comparing our results with those of the other experimental and theoretical studies, we consider the sp-d hybridization between the Mn 3d level and the sp band states to be an important factor affecting the band structures at the L point in the Brillouin zone over the whole range of Mn concentration.