A reverse osmosis treatment device includes: a first pressure vessel for treating an untreated water to produce a primarily treated water and a first permeated water; a second pressure vessel for treating the primarily treated water to produce a secondarily treated water and a second permeated water; a first cleaning solution tank for storing a first cleaning solution for cleaning the first pressure vessel; and a second cleaning solution tank for storing a second cleaning solution for cleaning the second pressure vessel. Each of the first pressure vessel and the second pressure vessel has therein a reverse osmosis membrane element having a reverse osmosis membrane. The first cleaning solution tank is connected to the first concentrate outlet pipe of the first pressure vessel, and the second cleaning solution tank is connected to the inlet pipe for the primarily treated water of the second pressure vessel.