The present invention addresses the problem of providing: a silicon nitride sintered body and a circuit substrate which exhibit both high mechanical strength and high thermal conductivity; a silicon nitride powder used as a starting material for these; and a production method for the silicon nitride powder. Provided is a silicon nitride powder which has a specific surface area of 4.0-9.0 m 2 /g, a ² phase ratio of less than 40%, and an oxygen content of 0.20-0.95 mass%, said silicon nitride powder wherein: the frequency distribution curve obtained from a volume-based particle size distribution measurement performed using laser diffraction scattering has two peaks; the tops of the peaks are within the range of 0.4-0.7 µm, and within the range of 1.5-3.0 µm respectively; the ratio (the frequency of the top of the peak in the particle size range of 0.4-0.7 µm/the frequency of the top of the peak in the particle size range of 1.5-3.0 µm) of the frequencies of the tops of the peaks is 0.5-1.5; and the ratio (D50/D BET ) (µm/µm) of the median diameter (D50) (µm) obtained from the particle size distribution measurement, to the specific surface area equivalent diameter (D BET ) (µm) calculated from the specific surface area is at least 3.5. Also provided are: a silicon nitride sintered body and a circuit substrate which are obtained from the silicon nitride powder; and a production method for the silicon nitride powder.