An evaporative fuel-processing system for an internal combustion engine comprises an evaporative emission control system including a canister having an air inlet port formed therein for introducing atmosphere into the canister and accommodating an adsorbent for adsorbing evaporative fuel generated in the fuel tank. A charging passage connects between the canister and the fuel tank. A two-way valve is arranged across the charging passage. A purging passage connects between the canister and the intake system. A purge control valve is arranged across the purging passage. An ECU carries out leak-checking of the evaporative emission control system. A negative pressure responsive-type bypass valve is arranged across a bypass passage bypassing the two-way valve. A negative pressure responsive-type vent shut valve is disposed to open and close the air inlet port of the canister. The ECU controls the bypass valve and the vent shut valve to open and close, by means of negative pressure developed within the intake system.