The present invention is a blue phosphor that is represented by a general formula xSrO.yEuO.MgO.zSiO2, where 2.970≦x≦3.500, 0.006≦y≦0.030, and 1.900≦z≦2.100. This blue phosphor has a crystal structure that is essentially a merwinite structure, and the crystal structure has a unit cell volume of 714.8 Å3 or less. Or, in this blue phosphor, a peak appearing around 2θ=22.86 degrees in an X-ray diffraction pattern obtained by measurement of the blue phosphor using an X-ray with a wavelength of 0.773 Å has a one-fifth value width of 0.17 degrees or less. Furthermore, the present invention is a light-emitting device having a phosphor layer including the phosphor. A suitable example of the light-emitting device is a plasma display panel.