An optical element includes a substrate, and an antireflection film provided on a surface of the substrate. The antireflection film is a graded layer having a refractive index that is progressively decreased from the substrate side towards an outer surface of the antireflection film. The graded layer has a reflectivity characteristic occupying ⅔ of a usable wavelength range around the center of the usable wavelength range. Reflectivity of the graded layer at a maximum value of the reflectivity characteristic is a peak value equal to or less than 0.4% The graded layer does not have a maximal value not corresponding to the peak value. At least one of reflectivities at both ends of the usable wavelength range is equal to or less than half the reflectivity at the peak value.