In a telescope according to the present invention, defocus amount of an optical system constituting the telescope is calculated; whether or not the defocus amount is within a predetermined in-focus range obtained from a focal point adjusting ability of human eye is discriminated; and when the defocus amount exceeds the in-focus range as a result of the above discrimination, the optical system is driven. The optical system is driven in a shorter time than a time required for removing a defocus by human eye.
