In factories, automatic inspection is performed using images that are acquired using industrial cameras. In these industrial cameras, lenses with various focal lengths are used according to their intended purposes. It is important that the optical systems of industrial cameras have small outer diameters, close minimum object distances and high resolution. In an optical system with a long focal length, the movement of the moving group increases during focusing, and this movement makes it difficult to reduce the minimum object distance. In addition, to achieve higher resolution, it is important to suppress the degradation that occurs because of machining errors or assembly errors. To solve these problems, we developed a floating focus system in which the diaphragm is fixed, and we introduced a moving group that is held using a helicoid and a fitting structure.