The machining device is equipped with a fiber laser oscillator for oscillating laser light of a top hat shape, and a light collecting lens and a machining head for collecting the laser light of the top hat shape and emitting the laser light onto a machining target such that the beam diameter of the laser light of the top hat shape at a position where light intensity becomes the one corresponding to a machining threshold of the machining target is about three times the size of the beam diameter of laser light in a Gaussian mode at the same position, when the laser light in the Gaussian mode has a beam quality substantially the same as that of the laser light of the top hat shape.