Method for manufacturing nitride semiconductor single crystal substrate

[Problem] To provide a method for manufacturing a nitride semiconductor single crystal substrate with which generation of cracks can be suppressed by relaxing strain inside a grown nitride semiconductor single crystal and with which a high-quality nitride semiconductor single crystal substrate can be effectively obtained. [Solution] In an embodiment of the present invention, a method for manufacturing a nitride semiconductor single crystal substrate is provided, the method including: a step for preparing a template (10) obtained by growing a first nitride semiconductor single crystal layer (2) on a substrate (1) of a different type; a step for forming grooves (3), which are made up of a plurality of linear grooves, in the template (10) by subjecting the first nitride semiconductor single crystal layer (2) and the substrate (1) of a different type to grooving by radiating laser light and forming HAZ4 at the same time as the substrate (1) of a different type is subjected to grooving, HAZ4 being a region formed by heating the inner surfaces of the grooves (3) by radiating laser light; a step for growing a second nitride semiconductor single crystal layer (5) on the template (10) in which the grooves (3) have been formed; and a step for cutting out a nitride semiconductor single crystal substrate (6) from the second nitride semiconductor single crystal layer (5).