PROBLEM TO BE SOLVED: To provide a system of concentrating a solution which is altered and denatured by heating at high temperature like a food related solution such as fruit juice, liquid sugar, and honey without alternation and denaturation.SOLUTION: A concentration system 1 of a liquid to be treated W includes a reverse osmosis membrane device 3 as primary concentration means and a membrane distillation device 4 as secondary concentration means in the middle of a raw water line 2. A heat exchanger 5 is arranged between the reverse osmosis membrane device 3 and the membrane distillation device 4. In the concentration system 1, the liquid to be treated W is primary-concentrated at the reverse osmosis membrane device 3 and is separated into a primary-concentrated liquid W1 and a membrane permeable water W2. The primary-concentrated liquid W1 is secondary-concentrated at the membrane distillation device 4 and is separated into a secondary-concentrated liquid W3 as a final product and a distilled water W4.SELEC
