Using Langer's $Im F$ method, we discuss the temperature dependence of nuclear fission width in the presence of dissipative environments. We introduce a low cut-off frequency to the spectral density of the environmental oscillators in order to mimic the pairing gap. It is shown that the decay width rapidly decreases at the critical temperature, where the phase transition from super to normal fluids takes place. Relation to the recently observed threshold for the dissipative fission is discussed.

12 pages, Latex, Submitted to Physical Review C for publication, 3 Postscript figures are available by request from hagino@nucl.phys.tohoku.ac.jp