Conventionally, there is a technique that two optical disks with a thickness half that of a normal optical disk are fixed together to produce a high-density optical disk. There has been developed an optical disk device which is applicable to both a normal optical disk and such a high-density optical disk. For example, Japanese patent application laid-open No. 6-282866(1994) discloses an optical disk device with a transparent plate which can be releasably placed on the optical path of laser light to be projected so as to change the optical path length from an optical head to a recording plane of an optical disk between a normal optical disk and a high-density optical disk.
However, the optical disk device disclosed in Japanese patent application laid-open No. 6-282866(1994) has problems in that the mechanism for releasably positioning the transparent plate requires a separate driving source inside the optical disk device which prevents miniaturizing the device complicates the device structure and increases the number of parts.
On the other hand, in this optical disk device, an objective lens of the optical head is driven to perform the focusing operation to change the distance (working distance) from there to the optical disk. However, the distance from the transparent plate to the optical disk is fixed when the transparent plate is located in the optical path of laser light. Therefore, when the driving distance of the objective lens is greatly increased due to a great plane pitching of the optical disk, the objective lens may contact the transparent plate.