A period of time during which a laser beam moves through an inner region of a beam spot that ranges from a center of the beam spot to an area corresponding to a thermal energy of 44% with respect to a total thermal energy of the beam spot is set as a moving time tp. “c”, “ρ”, “λ”, “T”, “T0”, “A”, and “Pd” respectively represents a specific heat, a density, a thermal conductivity, a melting temperature, an environmental temperature, a light absorptance, and an energy density of the inner region. A time calculated from Equation (1) is set as a melting time tm. A plate thickness is represented by x and a value of tm/tp is represented by y. The value y is set between a lower limit value (Equation (2)) and an upper limit value (Equation (3)). tm=c×ρ×λ×π[(T−T0)/(2×A×Pd)]2  (1) y=0.0027e0.36x  (2) y=0.0026e0.4512x  (3)