To attain the simplification, the reduction of the diameter and the miniaturization of the structure of a lens back adjusting device.

The lens back adjusting device is provided with a cylindrical member 10 elongating in an optical axis direction, a lens frame 20 arranged at the inside of the cylindrical member 10 and to hold a lens and an adjusting mechanism to adjust a focal position by moving the lens frame 20 in the optical axis direction to the cylindrical member 10. The adjusting mechanism incorporates a guide hole 11a formed at the cylindrical member 10 so as to allow elongation in the optical axis direction, a projecting part 22 formed at the lens frame 20 so as to be inserted in the guide hole and an annular cam member 30 turnably attached to the outer periphery of the cylindrical member 10 and exerting a cam action in the optical axis direction on the projecting part 22. Thus, the reduction of the diameter and the miniaturization are attained by a simple structure, and the focal position is adjusted without rotating the lens.