a first light source emitting a first light with a wavelength;

a first light path changer changing a traveling path of the first light;

an objective lens focusing the first light and forming a light spot on a recording medium;

a diffraction unit including:

first and second central diffraction areas which divide at least a part of a central light area into two sub-areas in a first direction and diffract light to the two sub-areas;

first and second peripheral diffraction areas which divide a first peripheral light area into two sub-areas in a second direction and diffract light to the two sub-areas; and

third and fourth peripheral diffraction areas which divide a second peripheral light area into two sub-areas in a second direction and diffract light to the two sub-areas, the diffraction unit dividing the first light reflected by the recording medium into at least 6 light areas and diffracting the at least 6 light areas; and

a first photodetector including:

first and second light-receiving portions receiving the first light diffracted into a +1^st or −1^st order light by the first and second central diffraction areas and converting the first light into an electrical signal; and

third through sixth light-receiving portions receiving a +1^st order light and/or a −1^st order light diffracted by the first through fourth peripheral diffraction areas,

wherein the first direction is a radial direction and the second direction is a tangential direction

wherein the first light reflected by the recording medium is divided into the central light area and the first peripheral light area and the second peripheral light area, and

wherein the first and second peripheral light areas are at respective sides of the central light area.