PURPOSE: To certainly protect a tubular membrane from damage due to negative pressure, is a separation apparatus using the tubular membrane, by providing a valve opened to the atmosphere by negative pressure formed when the supply of a raw liquid is stopped to the top part of a piping system.

CONSTITUTION: A membrane separation apparatus has such a structure that modules M having tubular membranes inserted into a porous support pipes thereof are communicated by a header and mounted on a stand 1 and a raw liquid is supplied to each modules M through pipes 2, 20, 21 by a pump 3 and a concentrate is discharged through pipes 71, 70, 7 and, on the other hand, a permeate is taken out through a pipe 8. In this separation apparatus, air chambers 9 are provided to the top parts of the pipes 20, 70 and valves 10 opened when the pressures of the air chambers become negative are provided. Because air is accumulated in this air chambers 9 during the operation of the apparatus, the raw liquid does not stick to the valves 10 even if it is viscous or solidifiable and, because the valve is certainly operated, the deformation of the tubular membrane due to negative pressure can be prevented well.