An .alpha.-sialon-based oxynitride phosphor characterized in that the content of .alpha.-sialon represented by the general formula: M.sub.xS.sub.12-(m+n)Al.sub.(m+n)O.sub.nN.sub.16-n:Ln.sub.y (wherein M is at least one metal selected from among Li, Ca, Mg, Y or lanthanide metals excluding La and Ce, Ln is at least one lanthanide metal selected from among Ce, Pr and La or at least one lanthanide metal selected from among Eu, Dy, Er, Tb and Yb, 0.3.ltoreq.x+y<1.5, 0