A laser processing device forms a modified region in an object to be processed by converging ultrashort pulse laser light at the object and comprises a laser light source emitting the laser light, a converging optical system converging the laser light emitted from the laser light source at the object, and an aberration providing part imparting an aberration to the laser light converged at the object by the converging optical system. In an optical axis direction of the laser light, letting a reference aberration range be a range of a converging-induced aberration as an aberration occurring at a position where the laser light is converged as a result of converging the laser light at the object, the aberration providing part imparts a first aberration to the laser light such that the laser light has an elongated range longer than the reference aberration range in the optical axis direction as an aberration range and an intensity distribution in the optical axis direction with a continuous undulation in the elongated range.