PURPOSE: To provide the fastener used for underwater clothes and working clothes in a special gas room by allowing the fastener to hold liquid/air tightness.

CONSTITUTION: To a first and a second base walls 14, 34 of a first and a second belt bodies 10, 30, engaging pieces 2, 6 whose cross section is a U-shape are attached crosswise, a hollow inserting body 15 provided projectingly from a first base wall 14 and inserting/holding bodies 36, 36 provided projectingly from a second base wall 34 are arranged in a U-shape of the engaging pieces 2, 6, and when a sliding body is moved to the upper end position a piston is pushed by the sliding body, fluid pressure is a fluid line 9 is raised, and the hollow inserting body 15 is expanded so that it is pressed liquid/air tightly against the holding bodies 36, 36.