The invention relates to an optical lens system (39) which is suitable for use in an optical scanning device (15) for reading and/or writing an optically scannable record carrier (9). The lens system comprises a first lens or objective lens (45) and a second lens or auxiliary lens (47) which is small in comparison with the first lens and which, in operation, is positioned between the first lens and the record carrier. The lenses are arranged in a lens holder (61). According to the invention, the lens holder (61) and one of the two lenses (45, 47) are manufactured as one integrated part (63) from a transparent synthetic material by means of a moulding process, and the other lens is arranged in the integrated part as a separate component, with a chamber (85) enclosed by the lens holder and the two lenses communicating with the environment of the lens system via an air vent connection (93). The lens system thus comprises only two parts to be assembled, so that the assembling process is considerably simplified. The air vent connection prevents the occurrence of overpressure in the chamber that might result from a temperature increase of the lens system and cause unwanted mechanical deformations of the integrated part. In a preferred embodiment, the integrated part (63) comprises the second lens or auxiliary lens (47), and the air vent connection comprises at least one groove-like channel (93), which is formed in a surface of a lens-holding portion (69) provided in the integrated part for holding the objective lens (45).