A method of manufacturing a nitride semiconductor device includes the steps of: forming a division guide groove by applying a laser beam having a wavelength and energy density causing multiphoton absorption to a surface of a substrate having a group III nitride semiconductor layer grown on a major surface thereof; removing deposits from the surface of the substrate by applying a laser beam having the wavelength to the surface of the substrate at energy density causing substantially no multiphoton absorption on the substrate; and dividing the substrate along the division guide groove.