An optical device includes a recording medium for recording and reproducing information by light beam, a generator of writing and reproducing light beam, a floating member provided at least one floating shoe which produces high air pressure at that surface thereof which faces to the recording medium at an extremely small distance thereof, an optical lens mounted upon the floating member for focusing the light beam upon the surface of the recording medium, a detector of the floating height of the floating member above the recording medium, and optical system controller for controlling in response to the output from the floating height detector in such a way that the point at which the light beam is focused by the optical lens is within a predetermined range relative to the surface of the recording medium.