We have studied the optical response of ZnSe in the 1.5--5.3-eV photon-energy range at room temperature by spectroscopic ellipsometry. The measured dielectric-function spectra reveal distinct structures at energies of the ${\mathit{E}}_{0}$, ${\mathit{E}}_{0}$+${\mathrm{\ensuremath{\Delta}}}_{0}$, ${\mathit{E}}_{1}$, and ${\mathit{E}}_{1}$+${\mathrm{\ensuremath{\Delta}}}_{1}$ critical points (CP's). These data are analyzed on the basis of a simplified model of the interband transitions. The ${\mathit{E}}_{0}$-(${\mathit{E}}_{0}$+${\mathrm{\ensuremath{\Delta}}}_{0}$) structures are characterized by a three-dimensional ${\mathit{M}}_{0}$ CP, the ${\mathit{E}}_{1}$-(${\mathit{E}}_{1}$+${\mathrm{\ensuremath{\Delta}}}_{1}$) structures by a two-dimensional ${\mathit{M}}_{0}$ CP, and the ${\mathit{E}}_{2}$ structure by a classical Lorentzian oscillator (damped harmonic oscillator). The experimental data could not be explained within the framework of the one-electron approximation, since excitonic effects may profoundly modify the CP singularity structure. The model is thus made to account for the excitonic effects at these CP's; our results are in satisfactory agreement with the experiment over the entire range of photon energies. Dielectric-function-related optical constants of ZnSe, such as the refractive index, the extinction coefficient, and the absorption coefficient, are also presented and analyzed.