Abstract: A device for increasing the buoyancy of a human body in water includes an inflatable tube member (1), a coupling unit (4) including a first part (8) and a second part (11), the coupling unit together with the tube member being arranged to form an endless float ring when the device is in the active state, a gas cartridge (6) having a punctureable membrane, a penetration element (23) arranged to puncture the membrane, a gas duct extending between the first part of the coupling unit and the tube member, wherein the gas cartridge and the penetration element are mutually displaceable in the direction towards each other. A first coupling member (20) of the first part is pivotally arranged about a pivot between an inactive position and an actuating position, in which the first coupling member, direct or indirect, causing the mutual displacement of the gas cartridge and the penetration element.