The invention concerns a control valve (c) for internal gas pressure of a liquid tank co-operating with an orifice (10) provided in the upper part thereof, allowing through a gas flow outside the tank when the difference between the internal pressure and the external pressure reaches a predetermined threshold, and an inverse flow when the internal pressure of the tank is lower than the external pressure. The invention is characterised in that the valve comprises a body (1) whereof the lower part is configured to close the tank orifice (10) with a mass such that beyond said pressure differential value constituting the predetermined threshold, the body (1) is spaced apart from the orifice (10), said body (1) further comprising a through recess (4) arranged in the extension of the tank orifice (10) when they are in contact, said recess (4) being provided with closing means (3) capable of being activated when the differential pressure between the tank and outside atmosphere is positive.