To provide a gate for a pipe line, by which the reduction of the effective sectional area of the pipe line is prevented even when an upper membrane is brought down.

A lower membrane 40 and the upper membrane 42 are installed to pads 34A and 34B under a sealed state in a plane sheet shape, a section between the upper membrane 42 and the lower membrane 40 is supplied with air and the upper membrane 42 is erected, and permanent elongation is generated in the erected upper membrane 42 when air is discharged. Even when the elongation is generated, the upper membrane and the lower membrane sink into an indentation 44 between the pad 34A and the pad 34B by their own weight. Thus, the upper membrane 42 and the lower membrane 40 are housed into the indentation 44 from the upstream side or downstream side of the pipe line 32. Accordingly, even when the upper membrane 42 is brought down and a fluid such as water is passed into the pipe line, the reduction of the effective sectional area of the pipe line 32 can be avoided.