We analyze explicit CP violation in the Higgs sector of the next-to-minimal supersymmetric standard model which contains an additional gauge singlet field N. It is shown that there is no mixing among scalar and pseudoscalar Higgs fields in the two Higgs doublets, and scalar-pseudoscalar mixings could exist between two Higgs doublets and the singlet N, and between N itself. CP symmetry is conserved in the extreme limits of < N > >> v, < N > << v, and tan beta >> 1. In the region of < N > = O(v) and tan beta = O(1), large scalar-pseudoscalar mixings are realized, and this effect can reduce the lightest Higgs mass. The mass difference between no mixing and mixing case is about 10 - 30 GeV. The neutron electric dipole moment in this model is consistent with the present experimental upper limit provided that squark and gaugino masses are heavy enough of O(1) TeV.