A scanning mirror unit for scanning a beam comprises: at least one cantilever which is formed by bonding an elastic material and a piezoelectric material together and supported by a base at its one end; and a movable mirror which is supported at least by a free end of the cantilever at its peripheral part. When voltage is applied to the piezoelectric material, the free end of the cantilever moves in a prescribed direction to press and move the peripheral part of the movable mirror while leaving a central part of the movable mirror at substantially the same position to cause a tilt to the movable mirror. With such composition of the scanning mirror unit, a reflecting member having a sufficient thickness (e.g. some hundreds of microns) can be employed for the movable mirror, by which a high power laser beam effective for treatment of affected parts, etc. can be reflected finely and durably.