PURPOSE: To recover the flexibility of a membrane by treatment to make the membrane hydrophilic easily and efficiently with less consumption of a chemical agent, labor, and time when the membrane has become hydrophobic and its flexibility has been reduced at the time of starting operation after a long time of stopping after the exchange of membranes or washing with the chemical agent.

CONSTITUTION: A means is installed to inject a hydrophilic agent on the transmitted liquid side 3 of a membrane 1A. The membrane 1A is made hydrophilic by the injected hydrophilic agent reaching the surface and inside of the membrane 1A while the hydrophilic agent being diluted by diffusion. The treatment can be completed with a chemical agent of the amount 10-100% of the capacity of the transmitted liquid side. The consumption of the chemical agent, labor, and time required for the treatment and the amount of waste liquid to be disposed of can be reduced remarkably.