Silicon nitride substrate, silicon nitride-metal composite, silicon nitride circuit board, and semiconductor package

A silicon nitride substrate comprising silicon nitride and magnesium,

• wherein when a surface of the silicon nitride substrate is analyzed with an X-ray fluorescence spectrometer under the following Condition I, XB/XA is 0.80 or more and 1.00 or less,
• (Condition I)
• an amount of magnesium (% by mass, in terms of oxide) at an intersection A of diagonals on any one surface of the silicon nitride substrate is defined as XA (%), the amount of magnesium obtained by analyzing the intersection A with the X-ray fluorescence spectrometer, and an arithmetic mean value of amounts of magnesium (% by mass, in terms of oxide) at four points of B1, B2, B3, and B4 which are on the diagonals and positioned 3 mm inward from corners of the silicon nitride substrate to a direction of the intersection A is defined as XB (%), the arithmetic mean value of the amounts of magnesium obtained by analyzing the four points B1, B2, B3, and B4 with the X-ray fluorescence spectrometer.