Optical reading method and optical reading system

An optical reading system comprising:

• a rotation driving unit for rotating an optical recording medium in which a distance from a light incident surface to an information recording layer is less than 100 μm and a distance from the light incident surface to the information recording layer which is the nearest from the light incident surface is 80 μm or less;
• a laser beam source for generating a laser beam;
• an objective lens for collecting the laser beam to irradiate the optical information recording medium with;
• a photoelectric conversion unit for receiving reflected light of the laser beam and converting it into an electrical signal;
• and
• a PRML processing unit for reading a signal by a PRML detection method on the basis of the electrical signal, wherein
• a constraint length n in the PRML processing unit is set to an integer which satisfies 0.5×(λ/NA)/(LV/f)−1<n<0.5×(λ/NA)/(LV/f)+1, wherein λ represents a wavelength of the laser beam, NA represents a numerical aperture of the objective lens, LV represents a linear velocity of the optical recording medium when recording, and f represents a clock frequency of a reading system.