Objective lens optical system and light beam splitting element

An objective lens optical system includes a light beam splitting element and an objective lens and is configured to focus a light beam with a wavelength lambda on an information recording surface of a first optical recording medium as a dual/double layer disc having a transparent substrate with a thickness t 1 and to focus a light beam with a wavelength lambda on an information recording surface of a second optical recording medium as a dual/double layer disc having a transparent substrate with a thickness t 2 (t 1< t 2 ). The light beam splitting element is sectioned into regions that are designed to focus a light beams on an information recording surface of either one of the first optical recording medium and the second optical recording medium. Either one region has a negative or positive power so that a light focus position of a light beam having passed through the first region by the objective lens and a light focus position of a light beam having passed through the second region by the objective lens are different.