A group III nitride crystal substrate, wherein anyone of the following conditions (i) to (iii) and the following condition (iv) are satisfied: (i) a plane spacing of arbitrary specific parallel crystal lattice planes of said crystal substrate being obtained from X-ray diffraction measurement performed with variation of X-ray penetration depth from a main surface of said crystal substrate while X-ray diffraction conditions of said specific parallel crystal lattice planes of said crystal substrate are satisfied, a uniform distortion at a surface layer of said crystal substrate represented by a value of |d 1 - d 2 |/d 2 obtained from a plane spacing d 1 at said X-ray penetration depth of 0.3 μm and a plane spacing d 2 at said X-ray penetration depth of 5 μm is equal to or lower than 1.9 × 10 -3; (ii) on a diffraction intensity profile of arbitrary specific parallel crystal lattice planes of said crystal substrate being obtained from X-ray diffraction measurement performed with variation of X-ray penetration depth from a main surface of said crystal substrate while X-ray diffraction conditions of said specific parallel crystal lattice planes are satisfied, an irregular distortion at a surface layer of said crystal substrate represented by a value of |v 1 - v 2 | obtained from a half value width v 1 of a diffraction intensity peak at said X-ray penetration depth of 0.3 μm and a half value width v 2 of the diffraction intensity peak at said X-ray penetration depth of 5 μm is equal to or lower than 130 arcsec; (iii) on a rocking curve being measured by varying an X-ray penetration depth from a main surface of said crystal substrate in connection with X-ray diffraction of arbitrary specific parallel crystal lattice planes of said crystal substrate, a plane orientation deviation of said specific parallel crystal lattice planes of said surface layer of said crystal substrate represented by a value of |w 1 - w 2 | obtained from a half value width w 1 of a diffraction intensity peak at said X-ray penetration depth of 0.3 μm and a half value width w 2 of the diffraction intensity peak at said X-ray penetration depth of 5 μm is equal to or lower than 350 arcsec; and (iv) said main surface having a plane orientation inclined in a <10-10> direction at an angle equal to or greater than 10° and equal to or smaller than 80° with respect to one of and planes of said crystal substrate. (i) a plane spacing of arbitrary specific parallel crystal lattice planes of said crystal substrate being obtained from X-ray diffraction measurement performed with variation of X-ray penetration depth from a main surface of said crystal substrate while X-ray diffraction conditions of said specific parallel crystal lattice planes of said crystal substrate are satisfied, a uniform distortion at a surface layer of said crystal substrate represented by a value of |d 1 - d 2 |/d 2 obtained from a plane spacing d 1 at said X-ray penetration depth of 0.3 μm and a plane spacing d 2 at said X-ray penetration depth of 5 μm is equal to or lower than 1.9 × 10 -3; (ii) on a diffraction intensity profile of arbitrary specific parallel crystal lattice planes of said crystal substrate being obtained from X-ray diffraction measurement performed with variation of X-ray penetration depth from a main surface of said crystal substrate while X-ray diffraction conditions of said specific parallel crystal lattice planes are satisfied, an irregular distortion at a surface layer of said crystal substrate represented by a value of |v 1 - v 2 | obtained from a half value width v 1 of a diffraction intensity peak at said X-ray penetration depth of 0.3 μm and a half value width v 2 of the diffraction intensity peak at said X-ray penetration depth of 5 μm is equal to or lower than 130 arcsec; (iii) on a rocking curve being measured by varying an X-ray penetration depth from a main surface of said crystal substrate in connection with X-ray diffraction of arbitrary specific parallel crystal lattice planes of said crystal substrate, a plane orientation deviation of said specific parallel crystal lattice planes of said surface layer of said crystal substrate represented by a value of |w 1 - w 2 | obtained from a half value width w 1 of a diffraction intensity peak at said X-ray penetration depth of 0.3 μm and a half value width w 2 of the diffraction intensity peak at said X-ray penetration depth of 5 μm is equal to or lower than 350 arcsec; and (iv) said main surface having a plane orientation inclined in a <10-10> direction at an angle equal to or greater than 10° and equal to or smaller than 80° with respect to one of and planes of said crystal substrate.