A reverse osmosis membrane separation device includes: a reverse osmosis membrane module; a flow rate detecting unit configured to detect a flow rate of permeate water to output a detected flow rate value corresponding to the flow rate; a pressure pump configured to be driven at a rotation speed corresponding to an input drive frequency and to feed supply water to the reverse osmosis membrane module; an inverter configured to output a drive frequency corresponding to an input current value signal to the pressure pump; and a control unit configured to calculate a drive frequency of the pressure pump by a velocity type digital PID algorithm, such that a detected flow rate value output from the flow rate detecting unit becomes a target flow rate value that is set in advance to output a current value signal corresponding to a calculation value of the drive frequency to the inverter.