Abstract: A process for sterilizing a filtration device which comprises employing therein a hollow fiber type module having hollow fibers of heat resistant polymer with a heat distortion temperature of 70.degree. C. or higher under a load of 18.6 kg/cm.sup.2 and with a pore diameter of less than 0.45 .mu.m and having a difference in average linear expansion coefficient between the sealing material for fixing the end portions of the hollow fibers in the module case, at a temperature of 20.degree.-121.degree. C. of 7.times.10.sup.-5 /.degree. C. or lower, and passing steam from the raw water side of the module to the filtrate side thereof, while withdrawing all or a part of the raw water and filtrate which partially fill the module or alternatively after withdrawal of raw water and filtrate.