Silicon nitride substrate and silicon nitride substrate production method

A silicon nitride substrate, comprising:

• a major layer and an intermediate layer as observed in dark-field with an optical microscope in a cross section of the silicon nitride substrate along a thickness direction of the silicon nitride substrate, the intermediate layer being darker than the major layer,
• a color difference between the major layer and the intermediate layer being not less than 0.1,
• m+1 of the major layers and m of the intermediate layers being alternately arranged in the thickness direction,
• a color difference between an nth major layer and an nth intermediate layer from one surface of the silicon nitride substrate and a color difference between a (n+1)th major layer and the nth intermediate layer from the one surface satisfying a first condition and a second condition for all values of n from 1 to m, n being an integer,
• the first condition being when there is a continuous region having a length of not less than 100 μm in a first direction in which 0.1≤CDna≤1.4, CDna being the color difference between the nth major layer and the nth intermediate layer from the one surface, the first direction being parallel to the cross section and perpendicular to the thickness direction,
• the second condition being when there is a continuous region having a length of not less than 100 μm in the first direction in which 0.1≤CDnb≤1.4, CDnb being the color difference between the (n+1)th major layer and the nth intermediate layer from the one surface.