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 m2/g, a beta 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 [mu]m, and within the range of 1.5-3.0 [mu]m respectively; the ratio (the frequency of the top of the peak in the particle size range of 0.4-0.7 [mu]m/the frequency of the top of the peak in the particle size range of 1.5-3.0 [mu]m) of the frequencies of the tops of the peaks is 0.5-1.5; and the ratio (D50/DBET) ([mu]m / [mu]m) of the median diameter (D50) ([mu]m) obtained from the particle size distribution measurement, to the specific surface area equivalent diameter (DBET) ([mu]m) 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.