PURPOSE: To provide air in an amount corresponding to the request for purging of a large amount by providing, in a predetermined portion of the atmospheric- port side of a canister, a means for varying the area open to the atmosphere by which the area open to the atmosphere of the canister is varied to match the amount of purging, and increasing the area open to the atmosphere when the amount by which the engine is purged is large.

CONSTITUTION: This evaporative fuel processing device 30, which prevents vapors vaporized from a fuel tank 21 from escaping into the atmosphere, has a charcoal canister 22 which is provided with a tank port 31 for connection to a vapor collecting pipe 25; a purge port 32 for connection to a vapor recirculation pipe 27; activated charcoal 33 adsorbing the vapors; and an atmospheric port 38. A solenoid switching valve 50 is mounted on the end of the atmospheric port 38, and a buffer canister 24 provided with a second atmospheric port 37 on its side open to the atmosphere is connected to the atmosphere side of the valve 50. The opening of the solenoid switching valve 50 is controlled by a solenoid valve control device 60 to those matching fuel supply time, driving time, parking time, and purging time during driving.