Abstract: There is provided a method for controlling an RO system, the method being capable of reducing power consumption (that is, CO2) and the amount of waste by reducing the amount of use of chemicals and the number of times of exchanging a membrane and capable of enabling stable operation and contributing to energy saving. A method for controlling an RO system, the RO system including a plurality of RO apparatuses 41 to 44 arranged in parallel and a control unit controlling a start/stop process, the start/stop process including an operation process and a stop process for the RO apparatuses 41 to 44, wherein control is performed so that the number of times of start/stop is larger for an RO apparatus that easily recovers treatability by start/stop.