It is desirable to greatly reduce the disturbance that arises in a focusing error signal by a astigmatic detection method and offset that arises in the tracking error signal by a push-pull method when reproducing various information storing disks, such as a DVD-RAM disk. It is also desirable to provide an optical disk apparatus which is able to reproduce a variety of high density optical disks, such as DVD-RAM and DVD-ROM disks, as well as conventional disks, such as CD, CD-ROM, CD-R disks, by using a simple optional head. For this purpose, an optical disk apparatus irradiates three optical beams produced by a diffraction grating on the specified position of an optical disk and detects reflected light using a specified optical detector divided into twelve parts.