A laser by which the length of the light path between the laser and a laser beam machine can be shortened. The apparatus is provided with a laser resonator and reflection mirrors which reflect the laser beam emitted from the laser resonator and cause the beam to go backward. The laser beam emitted from the output mirror of the laser resonator and reflected by the reflection mirrors travels on the optical path of a given length from the output mirror, and is outputted through the back plate of the laser. The direction in which the laser beam is outputted from the laser may sometimes be opposite to that in which the laser beam is emitted from the output mirror. One of the additional reflection mirrors can be a phase-delay reflection mirror in order to transform the laser beam into a circular polarization beam in the laser. <IMAGE>
