A silicon nitride substrate comprising silicon nitride crystal grains and a grain boundary phase and having a thermal conductivity of 50 W/m·K or more, wherein: the silicon nitride substrate has a thickness T1 of 1.0 mm or less, when an arbitrary sectional structure of the silicon nitride substrate is observed by scanning electron microscopy, the ratio (T2/T1) of the total length T2 of the grain boundary phase in a thickness direction with respect to the thickness T1 of the silicon nitride substrate is 0.01 to 0.30, when the dielectric strength of the silicon nitride substrate is measured at five measurement locations including a point S1 that is a point of intersection between diagonal lines of the silicon nitride substrate, and four points S2 to S5 that are midpoints between the point S1 and the respective corner portions of the silicon nitride substrate, the dielectric strength mean value is 15 kV/mm or more, the variation from the dielectric strength mean value when measured by a four-terminal method in which electrodes are brought into contact with the front and the rear surfaces of the silicon nitride substrate is 20% or less, when ρv1 is the volume resistivity value of the silicon nitride substrate having a voltage of 1000 V applied thereto at room temperature (25°C), and pv2 is the volume resistivity value of the silicon nitride substrate having a voltage of 1000 V applied thereto at 250°C, the ratio (ρv2/ρv1) is 0.20 or more, the average grain diameter with respect to a long diameter of the silicon nitride crystal grains is between 1.5 and 10 μm, and the grain boundary phase contains one or more kinds selected from rare earth elements, magnesium, titanium and hafnium as a sintering aid in a total amount of 2 to 14% by mass in terms of oxide content. the silicon nitride substrate has a thickness T1 of 1.0 mm or less, when an arbitrary sectional structure of the silicon nitride substrate is observed by scanning electron microscopy, the ratio (T2/T1) of the total length T2 of the grain boundary phase in a thickness direction with respect to the thickness T1 of the silicon nitride substrate is 0.01 to 0.30, when the dielectric strength of the silicon nitride substrate is measured at five measurement locations including a point S1 that is a point of intersection between diagonal lines of the silicon nitride substrate, and four points S2 to S5 that are midpoints between the point S1 and the respective corner portions of the silicon nitride substrate, the dielectric strength mean value is 15 kV/mm or more, the variation from the dielectric strength mean value when measured by a four-terminal method in which electrodes are brought into contact with the front and the rear surfaces of the silicon nitride substrate is 20% or less, when ρv1 is the volume resistivity value of the silicon nitride substrate having a voltage of 1000 V applied thereto at room temperature (25°C), and pv2 is the volume resistivity value of the silicon nitride substrate having a voltage of 1000 V applied thereto at 250°C, the ratio (ρv2/ρv1) is 0.20 or more, the average grain diameter with respect to a long diameter of the silicon nitride crystal grains is between 1.5 and 10 μm, and the grain boundary phase contains one or more kinds selected from rare earth elements, magnesium, titanium and hafnium as a sintering aid in a total amount of 2 to 14% by mass in terms of oxide content.