Excitonic luminescence from ZnS epitaxial layers has been studied under magnetic fields up to 9 T in Faraday configuration. The luminescence line due to the radiative recombination of light-hole free excitons slightly shifted to the lower-energy side (about 0.3 meV at 9 T). On the other hand, the luminescence line due to the radiative recombination of biexcitons split into two components. This splitting (about 1.2 tneV at 9 T) originated from the Zeeman splitting of light-hole free excitons in the final state of the optical transition for biexciton luminescence.