The present invention relates to a plasma display device apt to increase luminance of a phosphor layer and prevent degradation of a discharge characteristic, and to a phosphor used for the device. This plasma display device has a green phosphor having a crystal structure of Zn.sub.2SiO.sub.4:Mn, and a monovalent oxide is substituted for part of the green phosphor. The monovalent oxide is one or more of lithium oxide (Li.sub.2O), sodium oxide (Na.sub.2O), potassium oxide (K.sub.2O), cesium oxide (Cs.sub.2O), rubidium oxide (Rb.sub.2O), copper oxide (Cu.sub.2O), and silver oxide (Ag.sub.2O). This structure allows reduction of oxygen defects occurring in the green phosphor, suppression of the luminance decrease of the green phosphor, and improvement of a discharge characteristic.