This water treatment unit (3) can be used in a water treatment device that treats water using a reverse-osmosis membrane. Said water treatment unit (3) is provided with a casing (30), a pleated separation membrane (31) mounted inside said casing (30), a reinforcement member that is mounted to the separation membrane (31) and serves to reinforce said separation membrane, a rotation mechanism (42) that rotates the separation membrane (31), and a cleaning device (41) that is capable of cleaning the separation membrane (31). The separation membrane (31) has a plurality of island parts and a plurality of fibriform parts that extend from the island parts, are thinner than the island parts, and resemble fibers. The area of the membrane surface taken up by the fibriform parts is made larger than the area taken up by the island parts. A water treatment device is provided with the following: a first water treatment unit that has the above architecture and is capable of pretreating water to be treated; and a second water treatment unit that is capable of performing a main treatment on the water to be treated.