An object of the present invention is to provide a water treatment system in which explosions can be prevented even when filtering water to be treated that is able to generate flammable gas. A water treatment system according to an embodiment of the present invention includes a water-to-be-treated tank which stores water to be treated, a crossflow-type filtration membrane module which filters the water to be treated, a supply passage through which the water to be treated is supplied to the filtration membrane module from the water-to-be-treated tank by using a supply pump, and a recirculation passage through which the water to be treated is recirculated from the filtration membrane module to the water-to-be-treated tank. The supply passage is provided with an aspirator which mixes a gas with the supplied water to be treated, the gas being recirculated through the recirculation passage to the water-to-be-treated tank together with the water to be treated. The water-to-be-treated tank has an upper space provided above the liquid surface of the stored water to be treated, the upper space being hermetically filled with an inert gas. The water-to-be-treated tank is further provided with a gas transport passage through which the inert gas is supplied from the upper space to the aspirator.