In a device for recording or reproducing information by using a light beam, there is provided an optical disk comprising a base plate, a recording layer formed on the base plate, tracks provided for recording information on the recording layer, and a focus check area for checking if the light beam is focused at the optimum position on the recording layer when an optical head which generates the light beam faces the recording layer. Accordingly, it is possible to change the focus position for the recording layer of the optical disk by the objective lens of the optical head during recording and reproducing information. Also, it is possible to set it at a position where the bit error rate is smallest and has a wider margin for the shaft of the adjust position.