There is provided an objective optical system used for information recording/reproducing for three types of optical discs. The objective optical system includes an objective lens, and a diffraction structure formed on an optical surface. The diffraction structure includes a first area for contributing to converging the third light beam. The first area includes first and second steps defined by first and second optical path difference functions, respectively. The first step is configured such that diffraction orders at which diffraction efficiencies for the first, second and third light beams are maximized are 1st order, 0-th order and 0-th order, respectively. The first step satisfies a condition: −0.36×102