There is provided a nitride crystal substrate constituted by group-III nitride crystal, containing n-type impurities, with an absorption coefficient α being approximately expressed by equation (1) by a least squares method in a wavelength range of at least 1 μm or more and 3.3 μm or less.








α
=


N
e

⁢
K
⁢

λ
a

⁢


(



where

1.5
×

10

-
19



≤
K
≤

6.
×

10

-
19




,

a
=
3


)



,




(
1
)









here, a wavelength is λ (μm), an absorption coefficient of the nitride crystal substrate at 27° C. is α (cm−1), a carrier concentration in the nitride crystal substrate is Ne (cm−3), and K and a are constants, wherein an error of an actually measured absorption coefficient with respect to the absorption coefficient α obtained from equation (1) at a wavelength of 2 μm is within +0.1α, and in a reflection spectrum measured by irradiating the nitride crystal substrate with infrared light, there is no peak with a peak top within a wavenumber range of 1,200 cm−1 or more and 1,500 cm−1 or less.