A water treatment device includes a

A water treatment device includes a primary unit having a plurality of primary elements as reverse osmosis membrane devices disposed in parallel with each other to separate water to be treated into primary condensed water and fresh water; a pump which feeds the water to be treated from the upstream side of the primary unit to the primary unit; a secondary unit having secondary elements which are provided to be fewer in number than the primary elements and separate the primary condensed water into secondary condensed water and fresh water; a sub-element which separates one of the water to be treated and the primary condensed water; and a mode switching unit which switches the sub-element between a primary mode of being used as the primary element in the primary unit and a secondary mode of being used as the secondary element in the secondary unit.