To provide a concentrator for a surfactant solution capable of suppressing the generation of bubbles in a liquid storage tank.

This concentrator is provided with a membrane module 8 concentrating a surfactant-containing solution, a means 14 measuring a liquid amount, and a liquid storage tank 3 in which the solution is stored while the liquid amount is being controlled by the means 14, so that the solution is fed from the liquid storage tank 3 to the membrane module 8 and often concentration, the resultant concentrate 10 is returned to the liquid storage tank 3. The concentrator is further provided with a means 15 jetting a gas inert to the solution to the liquid surface of the liquid storage tank 3.