An optical reading system is provided to improve read signal quality with increasing its linear recording density. An optical recording medium in which the distance from a light incident surface to an information recording layer is less than 100 μm is irradiated with a laser beam through an objective lens and information stored in the information recording layer is read by means of a PRML detection method. At this time, the constraint length n in the PRML detection method is set to an integer which satisfies 0.5×(λ/NA)/(LV/f)−1