Optical bistability derived from the negative nonlinear absorption effect was investigated in 470 ppm erbium-doped fibers using a 1.5 µm laser diode. Optical bistability was observed in a laser intensity range of 0.3 W/cm2 to 2.5 kW/cm2 at a fiber length of 6 m. The transmitted laser intensity (I out), propagating through the fiber, decreased exponentially in accordance with the well-known exponential law. However, the amplitude of the optical bistability (ΔI out) increased logarithmically as the fiber length was increased. It was clarified that ΔI out was proportional to the product of exp (-L) and log (L) as a function of the fiber length (L). The optical bistability can be explained by considering an enhanced absorption model for a four-level system of the Er3+ ion.