To provide a sialon-based phosphor capable of forming a thin film by mixing it with a resin, and giving a uniform photoluminescent light with high light emission intensity, and to provide a method for producing a sialon-based phosphor not having excessively large agglomerated particles and a wide particle size distribution, there is provided a method for producing an α-sialon based phosphor represented by the formula:
MxLnySi12-(m+n)Al(m+n)OnN16-n
wherein M is at least one metal selected from Ca, Mg, Y and Li, Ln is at least one lanthanide metal selected from Eu, Dy, Er, Tb, Yb and Ce, ax+by=m (assuming that the valence of metal M is a and the valence of lanthanide metal Ln is b), 0<x≦1.5, 0.3≦m≦4.5, and 0<n<2.25), wherein the silicon nitride raw material used as the raw material is a silicon nitride raw material powder comprising amorphous silicon nitride and crystalline silicon nitride.