There may be provided a laser apparatus including: an optical resonator including an output coupler; a laser chamber containing a laser medium and disposed in an optical path inside the optical resonator; a pair of discharge electrodes disposed inside the laser chamber; an electrode gap varying section configured to vary a gap between the discharge electrodes; a laser beam measurement section disposed in an optical path of a laser beam outputted from the output coupler, the laser beam resulting from electric discharge between the discharge electrodes; and a controller configured to control the gap between the discharge electrodes through activating the electrode gap varying section, based on a beam parameter of the laser beam measured by the laser beam measurement section.