Abstract: An optical device may include a mirror for respectively reflecting and transmitting parts of a first laser beam as first reflected and first transmitted beams, and for respectively transmitting and reflecting parts of a second laser beam as second transmitted and second reflected beams; an optical system disposed so that the first and second laser beams are such that beam paths of the first transmitted and second reflected beams are parallel or substantially coincide, or such that beam paths of the first reflected and second transmitted beams are parallel or substantially coincide; first and second measuring units configured to respectively measure a beam parameter of the first transmitted or first reflected beams, and of the second reflected or second transmitted beams; and first and second adjusting units configured to adjust the first and second laser beams based on measurement results by the measuring units.