Abstract: The present concentration device includes a membrane module having a first and a second flow paths that are separated by a semipermeable membrane, the module including a U-shaped tube in which the semipermeable membrane is disposed, a first flow-path-inlet and a first flow-path-outlet connection parts, and a second flow-path-inlet and a second flow-path-outlet connection parts, the device including an introducing unit for introducing a subject solution with a predetermined pressure into the first flow path from the first flow-path-inlet connection part, separating the subject solution into a concentrate and a permeate through the semipermeable membrane, passing the permeate through the second flow path, draining the concentrate from the first flow-path-outlet connection part, introducing the subject solution with a pressure lower than the predetermined pressure into the second flow path from the second flow-path-inlet connection part, and draining a diluent diluted with the permeate, from the second flow-path