The head harness includes at least one expandable strap whose ends are connected to the mask for constituting a loop. The strap includes an element which is inflatable by pressurized gas for lengthening the strap up to a size sufficient for the user to locate the harness over his head. Pressurized gas is supplied to the element for increasing its size upon actuating of a manually controlled valve. Upon release of the valve, the strap contacts the head and maintains the mask. A selection valve authorizes venting of the inflatable element when in an "emergency" position and maintains a residual pressure in the inflatable element when in a "comfort" position. The harness or a storage box for the mask are designed for preventing storing of the mask in "comfort" position.