Method for producing silicon nitride substrate

A method for producing a silicon nitride substrate includes a raw material powder preparation step of preparing a raw material powder containing a silicon powder, a rare earth element compound, and a magnesium compound, wherein, when the amount of silicon in the raw material powder is expressed in terms of a silicon nitride content, the raw material powder contains the rare earth element compound at 1 mol % to 7 mol % in terms of an oxide content and contains the magnesium compound at 8 mol % to 15 mol % in terms of an oxide content; a sheet forming step of forming the raw material powder into a sheet article; a nitriding step of heating the sheet article in a nitrogen atmosphere at 1200° C. to 1500° C. and nitriding silicon contained in the sheet article; and a sintering step of sintering the sheet article under a nitrogen atmosphere after the nitriding step.