In order to obtain a high-power laser beam with a high beam quality with use of a small-sized and low-cost apparatus, there has been disclosed a laser resonator which is constructed so as to produce a single high-order transverse mode laser beam, and a technique for converting the laser beam with a single high-order transverse mode into a laser beam with a high beam quality. The laser resonator is provided with a regulating plate of a high-order transverse mode which is constructed so as to produce a laser beam in a single high-order transverse mode. A phase adjusting plate inverts a phase for each region of the laser beam divided depending upon a lateral mode thereof to match the phase for each region of the laser beam, and subsequently, a traveling direction of the laser beam is changed for each position thereof by means of the first binary optics element, and further, the laser beam is converted into a parallel beam by means of the second binary optical element. The first binary optics element changes the traveling direction of the ray of the laser beam for each region thereof so that the laser beam has a desired intensity distribution on the second binary optics element.