Disclosed are system and method for filtration capable of minimizing the damage of a filtration membrane module, reducing the energy consumption, and improving the filtration efficiency. The filtration method of the invention comprises performing a preliminary filtration of a feed water with first and second submerged-type filtration membrane modules; pressurizing an initial filtrate produced through the preliminary filtration with a pressure higher than an osmotic pressure of the feed water; filtering the pressurized initial filtrate with a reverse osmosis membrane module; interrupting the first submerged-type filtration membrane module performing the preliminary filtration; and backwashing the first submerged-type filtration membrane module caused to stop performing the preliminary filtration, wherein the preliminary filtration with the second submerged-type filtration membrane module is continuously performed while the first submerged-type filtration membrane module is backwashed.