A water treatment system according to an embodiment of the present invention includes a tank that stores water to be treated, a crossflow filtration membrane module that filters the water to be treated, a supply passage through which the water to be treated is supplied from the tank to the filtration membrane module by using a supply pump, and a recirculation passage through which the water to be treated, the water having passed through a membrane upstream side of the filtration membrane module, is recirculated to the tank. The tank has an upper space above a liquid surface of the stored water to be treated, the upper space being hermetically filled with an inert gas. The water treatment system further includes a gas transport passage through which the inert gas is introduced from the upper space into the supply passage or the filtration membrane module, and a compressor that is disposed in the gas transport passage and that pressurizes the inert gas. The compressor is preferably a water-sealed compressor.