To reduce the installation number of flow rate regulators and of opening/closing devices without spoiling advantages that membrane filtration operation can be carried out at several membrane filtration flow rates and membrane filtration operation can be carried out by combining connection of membrane modules of different size in a pressurized type membrane filtration apparatus which is constituted by connecting several membrane modules.

Check valves 17-1, 17-2 are provided at a downstream side of a retentate outlet of the membrane module and the opening/closing device 11 is provided after joining of flows therefrom. Further check valves 16-1, 16-2 are provided at a downstream side of a filtrate outlet or at a downstream side of a circulation water outlet and the opening/closing device 10 is provided after joining of flows therefrom or a check valve is provided at an upstream side of a raw water inlet. Thereby even when the using number of flow regulating valves and opening/closing valves are reduced, counter flow from a pipe branching point to other module system is prevented.