PURPOSE: To enable two-stage treatment by one pump, by raising the pressure of the liquid supplied to a first stage membrane module by the pressure required in the operation of a second stage membrane module.

CONSTITUTION: In a two-stage treatment process, operation pressures adequate to the respective membranes of a second stage membrane module 2 and a first stage membrane module 1 are respectively set to P2, P1. A pressure regulation valve 3 and a pump 4 are operated so that the supply pressure of a raw solution to the first stage membrane module 1 is set to P1+P2 and the pressure of the transmitted solution chamber 12 of the first stage membrane module 1 is set to P2. Liquid differential pressure P1 is acted on the membrane of the first membrane module 1 and liquid differential pressure P2 is acted on the membrane of the second membrane module 2 and permeation is performed in the membranes of the respective modules under a proper condition.