Heretofore, if the beam size of the resonance mode is made small in a long resonator length, there arises a problem that the operation of the laser resonator becomes easy to be unstable and hence the adjustment thereof becomes difficult. A laser resonance includes: a laser material; a telescope for magnifying a laser beam which has been made incident from the laser material and for reducing a laser beam which has been made incident from the side opposite thereto; a flat reflecting mirror for reflecting the laser beam which has been made incident from said telescope; a telescope for magnifying the laser beam which has been reflected by the flat reflecting mirror to be reduced by the telescope and to be amplified by the laser material to be made incident thereto; and a flat reflecting mirror for reflecting the laser beam which has been made incident from the telescope. The telescope having suitable magnification is selected for the resonator length, whereby the beam size of the resonance mode at the center of the resonator can be arbitrarily adjusted and the stable resonator which has the small beam size with the long resonator length can be constructed.