An evaporative fuel-processing system for an internal combustion engine includes a canister accommodating an adsorbent therein, for adsorbing evaporative fuel generated in a fuel tank of the engine, and a charging passage connecting between the canister and the fuel tank. A shutter valve is arranged in a fuel supply pipe of the fuel tank in the vicinity of an inlet end thereof, for isolating the interior of the fuel supply pipe from the atmosphere, and the shutter valve is disposed to be opened by the tip of a refueling gun when the latter is inserted into the fuel supply pipe. A pressure-intake port section is arranged in the fuel supply pipe at a side of the shutter valve closer to the main body of the fuel tank, for taking in pressure within the fuel supply pipe. A diaphragm valve is arranged in the charging passage, which has a casing, a diaphragm defining the interior of the casing into a first chamber and a second chamber, a valve element displaceable together with the diaphragm, an evaporative fuel outlet disposed to be opened and closed by the valve element, and a spring urging the valve element in a direction of closing the evaporative fuel outlet. The first chamber communicates with the pressure-intake port section, and the second chamber communicates with the interior of the fuel tank.