Patent ID: 6987723

Claim:
An optical disk device comprising: a rotation driving means for driving rotation of an optical recording medium comprising a substrate formed with a recessed portion and a projection portion in a surface thereof, an optical recording layer formed on the substrate at the surface formed with the recessed portion and the projecting portion and having a topography corresponding to the recessed portion and the projecting portion, and a light transmission type protection layer formed on the optical recording layer; a light source for emitting light for recording or playback having a wavelength of 405±5 nm to the optical recording layer; an optical system including an objective lens having a numerical aperture of 0.85±0.1 for focusing and emitting the light from the protection film side to the optical recording layer; a light receiving element for receiving a return light reflected at the optical recording layer, the light receiving element including four square-shaped adjacent light detection elements each having one corner contacting all of the other three light detection elements, such that the return light is received as one main spot by all four light detecting elements with a center of the one main spot arranged at said one corner of each of the light detection elements; and a signal processing circuit for generating a predetermined signal based on the return light received by the light receiving element wherein only the light for an underside of the recording layer in the recessed portion and a top side of the optical recording layer of the projecting portion is used as-a recording area; and the optical recording medium has a distance between a center of one projecting portion and a center of an adjacent projecting portion used as the recording area of 0.32±0.01 μm or a distance between the center of one recessed portion and the center of an adjacent recessed portion used as the recording area of 0.32±0.01 μm and has a depth of the recessed portion with respect to the projecting portion in a range of 19 to 24 nm.