To provide a watertight device with increased flexibility to meet a shape as much as possible and to fit an attached part because an existing watertight device has an operating part with less freedom in shape or insufficient in flexibility.

A partition to separate open and blocked regions is formed out of a flexible sheet material with water-rightness. Flexible and expandable tubes are used for an opening and closing mechanism. The tubes are fixed to the sheet material, and the adjacent tubes are adhered with two slide fasteners which control positions of the tubes. The tubes are expanded by adding pressure from inside as air or water is put into the tubes, and adjacent tubes are pressed to come into contact with each other. As a result, a watertight fastener to assure water-tightness is made.