The invention provides a magnesia powder which gives off ultraviolet light with a wavelength of about 250 nm in high efficiency when being aroused by ultraviolet light generated by gas discharge of Xe gas. Chlorine content of magnesia powder containing chlorine ranges from 0.005 to 10 quality percent. The magnesia in the gross amount besides chlorine has a purity of above 99.8 quality percent and BET specific surface area ranges from 0.1 to 30 m<2>/g.