An agent for increasing the rejection of a nanofiltration membrane or a reverse osmosis membrane which comprises a compound having a polyalkylene glycol chain having a weight-average molecular weight of 2,000 to 6,000; a process for increasing the rejection of a nanofiltration membrane or a reverse osmosis membrane which comprises bringing the nanofiltration membrane or the reverse osmosis membrane into contact with an aqueous solution of a compound having a polyalkylene glycol chain which is obtained by diluting the agent for increasing the rejection of a nanofiltration membrane or a reverse osmosis membrane described above with water; a nanofiltration membrane or a reverse osmosis membrane exhibiting a rejection increased in accordance with the process; and a process and an apparatus for water treatment using the nanofiltration membrane or the reverse osmosis membrane. The rejection and, in particular, the rejection to nonionic solutes, of a nanofiltration membrane or a reverse osmosis membrane can be increased while the flux is kept at a great value.