PURPOSE: To concel a difference between focus positions of beams having different wavelength without using a high performance lens whose color aberration is corrected by correcting the aberration with using image surface curvature.

CONSTITUTION: Light having wavelength at 830nm emitted from a semiconductor laser is made incident on an objective lens 4 and the recording surface of an optical disc 5 perpendicularly and forms a light spot on the recording surface of the optical disk 5. Light having wavelength at 780nm emitted from a second semiconductor laser forms a spot on the position apart from the objective lens by 0.5μm on the same path. Then, since the image surface curvature is 240h2μm, (h) is the height of an image (mm), the height of the image (h) is decided so that the curvature is equal to 0.5μm, thus, when inclination of the incident beam is 0° 39', the focus positions are coincident with each other. By this method the need for using a composite lens whose color aberration is corrected is eliminated, and the miniaturization of the objective lens and the reduction in weight are attained.