PURPOSE: To improve the degassing performance and deaerating performance by connecting selectively a pressure reduction line with one film module and a deaeration line with the other film module respectively by means of changeover operation of a second flow path changeover means and a third flow path changeover means.

CONSTITUTION: First and second film modules 1 and 2 formed by storing gas permeable films are connected together in series, and a first flow path changeover means 5 for changing over the raw liquid flowing direction to the film modules 1 and 2 is formed. Also a second flow path changeover means 8 for connecting selectively either of the first film module 1 or the second film module 2 with a pressure reduction line 6 extending from a pressure reduction suction means is formed. Further a third flow path changeover means 9 for connecting selectively either the first film module 1 or the second film module 2 with a deaeration line 7 for supplying inactive gas other than the gas to be deaerated is formed. The degassing performance and backwash performance can be improved by the arrangement to carry out the stabilized operation for a long time.