An optical pickup includes a light source which emits light, a grating which separates a portion of the light emitted from the light source, a reflecting member which reflects another portion of the light emitted from the light source, a monitoring photodetector disposed on a traveling path of the light reflected from the reflecting member and which measures the reflected light, an optical path changer which changes an optical path of the light separated by the grating, an objective lens light which condenses the light the optical path of which is changed onto a disc, and a signal detecting photodetector which receives the light reflected from the disc.