There is provided a separation membrane unit wherein a plurality of separation membrane subunits (11a-c) communicate with raw water lines A (9a-c) that comprise raw water supply valves W (10a-c), water discharge lines B (12a-c) that comprise water discharge valves X (13a-c), raw water lines C (16a-c) that comprise first raw water communication valves Y (17a-c), raw water lines D (19a-c) that comprise second raw water communication valves Z (20a-c), and permeate water lines E (21a-c); and wherein the raw water lines A communicate with the water discharge lines B, the raw water lines C communicate with the raw water lines D, the raw water lines A communicate with a main raw water line 7, the water discharge lines B communicate with a main water discharge line 14, the raw water lines C and the raw water lines D communicate with a collection line 18, and the permeate water lines E communicate with a main permeate water line 22.