The present invention relates to a method for manufacturing a silicon nitride substrate and, more specifically, comprises the steps of: forming a slurry by mixing silicon nitride powder, a ceramic additive, and a solvent; molding the slurry to form sheets; sandwiching at least one of the sheets between a lower plate and an upper plate to form a stacked structure; degreasing the stacked structure; and sintering the stacked structure. At least one of the lower plate and the upper plate comprises a plurality of protrusions provided on one surface thereof, and the protrusions extend in parallel to each other in one direction.