A GaN crystal substrate has a crystal growth surface on which a crystal is grown, and a rear surface opposite to the crystal growth surface. The crystal growth surface has a roughness Ra(C)of at most 10 nm, and the rear surface has a roughness Ra(R) of at least 0.5 μm and at most 10 μm. A ratio Ra(R)/Ra(C) of the surface roughness Ra(R) to the surface roughness Ra(C) is at least 50. Thus, a GaN crystal substrate of which front and rear surfaces are distinguishable from each other is provided, without impairing the morphology of a semiconductor layer grown on the GaN crystal substrate.