Patent ID: 6845066

Claim:
A distance-change detecting method for detecting a change of the distance between a second optical means and an optical recording medium in an optical system having a first optical means for condensing the light to irradiate the optical recording medium and the second optical means being set between the first optical means and the optical recording medium in order to realize a numerical aperture larger than that of the first optical means, the method comprising the steps of: detecting the light incident to the second optical means from the first optical means at an incidence angle at which a numerical aperture is larger than a predetermined value and reflected from a plane facing the optical recording medium out of the second optical means; detecting the distance change in accordance with the amount of the reflected light; calculating a light amount ratio between a high frequency component of the incident light and a high frequency component of the reflected light; and controlling a distance between the optical recording medium and the second optical means so that the distance between the disk and the second optical means is constant in a near-field range, wherein one of the light incoming to the first optical means and the light incoming to the second optical means is detected, and wherein the distance is controlled based on the calculated ratio.