An optical pickup apparatus according to an embodiment of the present invention, comprises an optical unit for projecting a plurality of wavelengths of light, the light including a celadon light, toward a recording medium and receiving the light reflected from the recording medium to thereby detect an information signal and an error signal, and a compatible objective lens for focusing a light projected from the optical unit to thereby form an optical spot on a recording surface of the recording medium and applying at least three light having different wavelength ranges to be compatibly used for both the high-density and the low-density recording mediums. Accordingly, the optical pickup apparatus is capable of compatibly employing a high-density recording medium, which uses a celadon light and is slimmer than a digital versatile disc (DVD), and at least one low-density recording medium.