To increase the rigidity of a supporting frame supporting an optical system in a laser treatment device.

The optical system supporting frame 9 is fitted to the laser treatment device irradiating a body to be treated 6 placed on a movable treated-body base plate 5 with laser beams 7. The optical system supporting frame has a bridge girder 9a with a disposal surface 90 disposing the optical system 2 introducing the laser beams 7 and the bridge girder 9b supporting the bridge girder 9a from a lower section on both end sides in the longitudinal direction. In the optical system supporting frame the bridge girder 9a has a rising section 91 upwardly extended while crossing the disposal surface 90 on both end sides in the cross direction. The rigidity can be increased by a small mass increase, and an effect enlarging the rigidity per a mass is obtained. Resonance frequencies of optical system bridge girders can be elevated by increasing the rigidity at a mass ratio, and the vibration of the optical system bridge girders by the reaction of a shifter operation can be suppressed.