The invention is characterized in that, in a silicon nitride substrate comprising a grain boundary phase with silicon nitride crystal particles and having a heat conductivity of 50W/m·K or greater, the sectional structure of the silicon nitride substrate has a ratio (T2/T1) of 0.01 to 0.30 between the thickness T1 of the silicon nitride substrate and the total length T2 of the grain boundary phase in the thickness direction, and a variation in the dielectric strength of 15% or less from the mean value, as measured by the four terminal method, in which electrodes are brought into contact with the front and the back of the substrate. In addition, a dielectric strength mean value of 15 kv/mm or greater is desirable. According to this constitution, the silicon nitride substrate having a small dielectric strength variation, and the silicon nitride circuit substrate using the same, are obtained.