PURPOSE:To improve mechanical durability of a pinch valve by forming a pair of opposite thin wall portions at the central part if a flexible tube to alternately produce Z-shaped and S-chaped bending seal through initial pressure contact with difference in positin of the thin wall portions on every pressure receiving. CONSTITUTION:A cylindrical body l has a take-in port 2 for pressurized air, and a concentric cylindrical flexible tube 3 is mounted in the interior of the body. Bothe ends of the tube 3 are blocked up airtightly at flange portions, and a pair of thin wall portions 3a, 3a vertically confronting each other are formed in the vicinity of the central poriton of the tube. When pressurized air is pressed into the take-in port 2 of a pinch valve, buckling is caused at thin wall portions having relatively lower strength than the other thick wall portions 3b so that the thin wall portions are pressed to each other. When the thin wall portions are further pressureized, the thin wall portions cause a difference in position to block up the tube all over the surface with the Z-shaped bending seal. After the pressurized air is discharged, on pressing the pressurized air into the take- in port. S-shaped bending seal is caused by redidual stress.