A buoyancy compensator having a locking system (27) that ensures the ballast (25) not to drop unexpectedly during diving and to removable by one hand of a scuba diver. The locking system (27) includes a releasing unit (30), a female locking unit (50) and a male locking unit (70). The releasing unit includes a turn leg (32) that is to be interposed between the female locking unit and the male locking unit. When the turn leg is forcedly turned at a certain angle clockwise or counterclockwise, an engagement of the male locking unit to the female locking unit is released.