Recently, fluorescence observation using further longer wavelength than that in a conventional microscope has become to be carried out, so that an optical system having excellent optical performance in near-infrared wavelength range has been desired. For example, in a multi photon observation such as a two photon observation, in order to observe fluorescence in visible light by exciting a sample with longer wavelength illumination light, a microscope objective lens that excellently corrects chromatic aberration both in near-infrared light and in visible light has been increasingly desired. Moreover, in application in an optical tweezers also, the optical system is necessary to be excellently corrected in near-infrared range as well as in visible light range. There has been known an objective lens whose chromatic aberration is corrected in wavelength range from 400 nm to 1000 nm disclosed in Japanese Patent Application Laid-Open No. 2006-065030.
However, since a bright observation image with high resolution is necessary in fluorescence observation, such an objective lens has been required to have a large numerical aperture. Accordingly, in a conventional objective lens, there has been a problem that a numerical aperture thereof is not sufficient.