The absorptive properties of the atomic transition in the Rb85 D2 (A) line (5S1/2-5Pa/2) at 780.0 nm are measured by using a continuous tunable diode laser and a CCD detector in Rb vapour cell under the condition of extremely weak incident intensity. When the intensity of incident light decreases from 420pW/cm2 to 0.03pW/cm2, the transmittance is found to be independent of the incident intensity within our experimental accuracy.