To provide a rotation adjusting mechanism of an optical element, which is not easily rotated even by an external force such as vibrations and shocks, and is freely rotatable and excellent in operability at the time of adjusting the optical element, and is small and light.

This is a rotation adjusting mechanism of an optical element, comprised of a holding part 2c which is turned about an axial line by a turning means and holds an optical element 1 at the middle part in the axial direction, a holding member 2 provided with a recessed part 2b on an end face in the axial direction, a supporting member 3 for supporting the end part to be turnable, on which the recessed part 2b of the holding member 2 is not formed, a spherical member 5 abutted on the recessed part 2b of the holding member 2, and an energizing member 4 which is abutted on the spherical member 5 and energizes the spherical member 5 to the side of the recessed part 2b of the holding member 2.