To display the distance of an object being observed by a worker to him in an optical apparatus to move a focus adjusting lens block for the adjustment of focus.

A collimation telescope as optical apparatus is provided with an eyepiece lens block 15 to observe an image formed by an objective optical system having a focus adjusting lens block 12. In this case, an AF sensor 21 is provided to detect a defocusing value of an object image by the objective optical system, a computation/control circuit 23 which determines the defocusing value through the AF sensor 21 to move the focus adjusting lens block 12 by driving an AF motor 31, a moving value detector 19 to detect a moving value of the focus adjusting lens block 12 as moved from an infinitive distant focusing position and an internal visual field display device 17. Now, the computation/ control circuit 23 computes the distance of the object based on the position of focus adjusting lens block 12 detected by the moving value detector 19 and the defocusing value detected through the AF sensor 21 and the distance of the object computed is displayed on the internal visual field display device 17.