of the light flux of the second semiconductor laser 201 in the same manner the beam expander 106 107 can conduct the chromatic aberration correction and the spherical aberration correction in fig1 the light flux passed through the beam expander 106 107 passes through the stop 108 under the finite diverging condition having the intensive diverging angle and is light converged on its third information recording surface through the third protective layer thickness t 3 1 1 u2212 1 3 mm preferably 1 2 mm of the third optical disk 110 u2033 by the objective lens 109 formed of only the refractive surface and the light converging spot is formed here then because the light flux modulated by the information pit and reflected on the third information recording surface passes through again the objective lens 109 stop 108 beam expander 107 106 second beam splitter 105 and collimator 104 reflected by the beam splitter 103 and successively reflected by the third splitter 203 after that the astigmatism is given by the cylindrical lens 204 the light flux passes through the sensor lens 205 and is incident on the light receiving surface of the light detector 206 by using its output signal the reading signal of the information recorded in the third optical disk 110 u2033 is obtained further by detecting the shape change of the spot and the light amount change by the position change on the light detector 113 the focusing detection or track detection is conducted.