Abstract: The laser machining device includes an observation image acquiring unit configured to repeatedly acquire an observation image of a machining spot of laser light emitted from a laser optical system to a street on a wafer while a machined groove is being formed, a luminance detector configured to detect a luminance of a plasma generated at the machining spot by emission of the laser light based on the observation image every time the observation image acquiring unit acquires the observation image, a correspondence information obtaining unit configured to obtain correspondence information indicating a correspondence relationship among the luminance, energy of the laser light and a machined state of the machined groove, and a machined state assessing unit configured to assess the machined state with reference to the correspondence information based on the luminance and known energy of the laser light every time the luminance detector detects the luminance.