To provide a laser axis adjusting device allowing a plurality of optical axes of laser beams to be matched with a high degree of accuracy in a short time, as well as achieving higher safety by rarely being in danger of getting laser beam in eyes.

The adjusting device is provided with a reflecting mirror device 10 allowing a plurality of laser beams 1a, 2a output from multiple laser oscillators to repeatedly be reflected to project them on the same detector plane. The reflecting mirror device 10 contains a first half mirror M1 in which a laser beam can enter from backside, a plurality of reflecting mirrors M2, M3 and M4 reflecting those laser beams reflected on the first mirror M1 many time and then returning them to the first mirror M1, and a movable mirror MA located on an optical path passed through and faced the first mirror M1. As the movable mirror MA deflects these laser beams from a reflected light path and reflects them on the next mirror M3, the reflected path at each of reflecting mirrors, M1, M2, M3 and M4 is significantly misaligned by every reflection. Therefore, two points on the second mirror M2 (or any other mirror) are adjusted to be a single point to match the plurality of optical axes.