Provided are a silicon nitride substrate made of a silicon nitride sintered body that is high in strength and thermal conductivity, a method of producing the silicon nitride substrate, and a silicon nitride circuit substrate and a semiconductor module that use the silicon nitride substrate.According to the silicon nitride sintered body, in a silicon nitride substrate consisting of crystal grains 11 of β-type silicon nitride and a grain boundary phase containing at least one type of rare earth element (RE), magnesium (Mg) and silicon (Si), the grain boundary phase consists of an amorphous phase 12 and a MgSiN2 crystal phase 13; the X-ray diffraction peak intensity of any crystal plane of a crystal phase containing the rare earth element (RE) is less than 0.0005 times the sum of the diffraction peak intensities of (110), (200), (101), (210), (201), (310), (320) and (002) of the crystal grains of the β-type silicon nitride; and the X-ray diffraction peak intensity of (121) of the MgSiN2 crystal phase 13 is 0.0005 to 0.003 times the sum of the X-ray diffraction peak intensities of (110), (200), (101), (210), (201), (310), (320) and (002) of the crystal grains of the β-type silicon nitride.