To carry out measuring of tilt angle of an objective lens with high precision while reducing the loss of light quantity when reading signal from a disk in the objective lens utilized for pickup of an optical disk such as DVD.

In this objective lens 10, a reflection preventing film that prevents reflecting of laser beam for reading is especially provided on a part (effective diameter) 11 on a first surface 10a where laser beam (wave length in the vicinity of 650 nm) is passed for reading the signal from an optical disk 1. A transmission preventing film that prevents transmission of laser beam that is a different wave length from the wave length 650 nm is provided in a part 12 that is outside the above mentioned effective diameter. On the other hand, a reflection preventing film to prevent the reflection of the laser beam of wave length of 650 nm is provided on a second surface 10b.