An evaporation fuel processing system is equipped with a vapor passage, a canister, a purging passage, a purge control valve, a filling amount detector, and an ECU. The ECU comprises an adsorption amount detecting circuit that detects the adsorption amount of an evaporation fuel, which is adsorbed by the canister upon filling of the fuel at a time when a fuel is filled into a fuel tank, a breakthrough time calculating circuit that calculates a time, based on the detected adsorption amount, at which the canister would experience breakthrough in the event that the evaporation fuel were not purged into an engine from the canister, and a control circuit that carries out purge processing by the purge control valve based on the calculated time.