Method of manufacturing crystal substrate and crystal substrate

A crystal substrate, comprising:

• a first crystal body which is a substrate comprising a single crystal of group III nitride produced by vapor phase method and having a first main surface, and in which c-plane of the single crystal is curved in a concave spherical shape with a predetermined curvature;
• and
• a second crystal body comprising a single crystal of group III nitride grown on the first main surface using a mixed melt containing an alkali metal and a group III element.

A crystal substrate which is a substrate comprising a single crystal of group III nitride, and in which c-plane of the single crystal is curved in a concave spherical shape with a radius of curvature of 10 m or more with respect to either one of both main surfaces of the substrate.