An optical disk drive (101) for optically recording and/or reproducing information signals is provided in which the power of a light beam emitted from an optical head (104) to an optical disk (102) is controlled by an optical-coupling efficiency varying elements (214, 215) correspondingly to the type of the optical disk, recording layer in a multilayer optical disk and a mode of operation selected while a variation of the optical-coupling efficiency is being detected by a light-detecting element (216), thereby positively varying the optical-coupling efficiency in a minimum necessary time. Thus, the power of the light beam focused on the optical disk can be varied in a wide range without having to extremely raise the ratio in output power between modes of operation at a light source (212). Therefore, even a light source whose rating of optical output power is small can be used to write and read information signals to and from any of optical disks of different types or to each of recording layers in a multilayer optical disk with a high accuracy. Namely, good characteristics in writing and reading information signals can be assured without having to largely vary the ratio in output power of the light source from the write to read mode, from one to another type of optical disks or from one to another recording layer of the multilayer optical disk.