An optical device, comprising: a mirror (30, 30A, 30B, 30C, 130, 130A) configured to reflect part of a first laser beam incident on one principal surface thereof as a first reflected beam (L2r, L2rm) and transmit the other part of the first laser beam through the mirror (30, 30A, 30B, 30C, 130, 130A) as a first transmitted beam (L2tm, L2t), and to transmit part of a second laser beam incident on the other principal surface thereof through the mirror (30, 30A, 30B, 30C, 130, 130A) as a second transmitted beam (L3t, L3tm) and reflect the other part of the second laser beam as a second reflected beam; an optical system (M1, M2) disposed so that the first and second laser beams (L2, L3) are incident on the mirror (30, 30A, 30B, 30C, 130, 130A) such that a beam path of the first transmitted beam (L2tm, L2t) and a beam path of the second reflected beam are made to be parallel to each other or to substantially coincide with each other, or such that a beam path of the first reflected beam (L2r, L2rm) and a beam path of the second transmitted beam (L3t, L3tm) are made to be parallel to each other or to substantially coincide with each other; a first measuring unit configured to measure a beam parameter of at least one of the first transmitted beam (L2tm, L2t) and the first reflected beam (L2r, L2rm); and a second measuring unit configured to measure a beam parameter of at least one of the second reflected beam and the second transmitted beam (L3t, L3tm); characterized by a first adjusting unit configured to adjust the first laser beam based on a measurement result by the first measuring unit; and a second adjusting unit configured to adjust the second laser beam based on a measurement result by the second measuring unit. a mirror (30, 30A, 30B, 30C, 130, 130A) configured to reflect part of a first laser beam incident on one principal surface thereof as a first reflected beam (L2r, L2rm) and transmit the other part of the first laser beam through the mirror (30, 30A, 30B, 30C, 130, 130A) as a first transmitted beam (L2tm, L2t), and to transmit part of a second laser beam incident on the other principal surface thereof through the mirror (30, 30A, 30B, 30C, 130, 130A) as a second transmitted beam (L3t, L3tm) and reflect the other part of the second laser beam as a second reflected beam; to reflect part of a first laser beam incident on one principal surface thereof as a first reflected beam (L2r, L2rm) and transmit the other part of the first laser beam through the mirror (30, 30A, 30B, 30C, 130, 130A) as a first transmitted beam (L2tm, L2t), and to transmit part of a second laser beam incident on the other principal surface thereof through the mirror (30, 30A, 30B, 30C, 130, 130A) as a second transmitted beam (L3t, L3tm) and reflect the other part of the second laser beam as a second reflected beam; an optical system (M1, M2) disposed so that the first and second laser beams (L2, L3) are incident on the mirror (30, 30A, 30B, 30C, 130, 130A) such that a beam path of the first transmitted beam (L2tm, L2t) and a beam path of the second reflected beam are made to be parallel to each other or to substantially coincide with each other, or such that a beam path of the first reflected beam (L2r, L2rm) and a beam path of the second transmitted beam (L3t, L3tm) are made to be parallel to each other or to substantially coincide with each other; a first measuring unit configured to measure a beam parameter of at least one of the first transmitted beam (L2tm, L2t) and the first reflected beam (L2r, L2rm); and a second measuring unit configured to measure a beam parameter of at least one of the second reflected beam and the second transmitted beam (L3t, L3tm); characterized by a first adjusting unit configured to adjust the first laser beam based on a measurement result by the first measuring unit; and a second adjusting unit configured to adjust the second laser beam based on a measurement result by the second measuring unit.