To provide a method for achieving high dehydration and concentration of a water-containing solvent at low running cost.

The water-containing solvent is evaporated in an evaporator 3 to generate water-containing solvent vapor. The water-containing solvent vapor is supplied into a non-permeating side chamber 13 of a dehydration membrane module 4, while inert gas (dehumidified at least either before introduced into a relay tank 5 or after drawn out of the relay tank) stored in the relay tank 5 is supplied into a permeating side chamber 14 of the dehydration membrane module 4. Water vapor having permeated the permeating side chamber 14 from the non-permeating side chamber 13 is purged with the inert gas after dehumidified, and the inert gas is returned to the relay tank 5 and recycled.