We report the results of ac susceptibility measurements on Rb3C60 fullerene superconductor prepared with C60 crystals. The temperature, Tirr, of the maximum in X” shifts to high temperature with increasing ac magnetic-field frequency f. We fit the irreversibility line (IL) Tirr(Hdc) with the scaling relation Hdc = Hirr(0)(1-Tirr/T0)n and obtain n = 1.5 and T0 = 30.3 K for the 10 kHz IL near Tc. We find a roughly linear dependence on logarithmic f for 1/Tirr. By comparing the results to the expected frequency responses for flux pinning and flux flow, the observed frequency dependence of Tirr is interpreted by thermally activated flux creep in the fullerene.