The intensity change of a photoluminescence (PL) due to the addition of a below-gap excitation (BGE) light reveals the properties of a below-gap state quantitatively. We observed a transient PL of Eu^<3+> due to the BGE in a Y_2O_2S:Eu^<3+> phosphor, and studied its Eu^<3+> concentration dependence. The result indicates that the trap center originates from a native defect of the host crystal, not from the Eu^<3+> incorporation.
