A reverse osmosis treatment apparat

A reverse osmosis treatment apparatus includes: a first unit including a plurality of parallel-arranged reverse osmosis membrane modules that perform a primary treatment on water to be treated; and a plurality of second units each including one or more parallel-arranged reverse osmosis membrane modules that perform a subsequent treatment on concentrated water separated at the first unit. Each of the second units includes a concentrated water supply pipe, a concentrated water discharge pipe, a supply pipe valve, and a discharge pipe valve. The concentrated water discharge pipes are connected to the concentrated water supply pipes through a concentrated water return pipe and return pipe valves. A reverse osmosis treatment method includes, after performing subsequent treatments through two or more second units, switching the second unit that performs the subsequent treatment at the last stage.