A fuel-vapor emission-control system for an internal combustion engine which is capable of achieving both a reduction of the exhaust of vapor into the atmosphere and the prevention of an excessive increase in pressure within the fuel tank, regardless of the amount of internal pressure in the fuel tank, by providing on an atmospheric release port of the canister which adsorbs vapor adsorbed from the fuel tank of the internal combustion engine, thereby preventing release of the vapor into the atmosphere, an atmospheric release surface area changing valve which changes the surface area of an aperture to the atmosphere. The atmospheric release surface area valve operates to make the atmospheric release surface area larger when the internal pressure in the fuel tank is large than when the internal pressure in the fuel tank is small. The atmospheric release surface area changing valve can be configured as a mechanical type of pressure-sensitive valve which opens when the internal pressure in the canister exceeds a set value, and can also be configured as an electromagnetic valve, the degree of opening of which can be electrically varied in response to a detected value of vapor pressure within the system to thereby change the atmospheric release surface area.