A means capable of simply and efficiently concentrating a cell suspension. A concentrator has a culture vessel having a first port and a second port, a server bag having a port, a case having a hollow fiber bundle in the internal space, a filtering device having an inlet port, a first outlet port, and a second outlet port, a collection vessel having a port, a liquid supply circuit connected to the first port, the inlet port, and the server bag's port so that flow passages are switchable, a liquid discharge circuit connected to the second port, the first outlet port, the second outlet port, and the collection vessel's port so that flow passages are switchable, a liquid supply mechanism having a switching mechanism, a supply pump, and a discharge pump, and a rotation mechanism rotating the filtering device.