PURPOSE: To suppress vaporization of fuel in a float chamber, by providing a control valve in which when an other vent passage is opened, generation of a stream of air flowing from the float chamber through said passage to a canister can be prevented.

CONSTITUTION: When an engine is in operation, a solenoid coil 15 is electrically conducted, and intake pipe negative pressure is fed to a diaphragm chamber 12 through a conduction pipe 21, then a valve element 9 is moved leftward against a spring 16 to close a valve port 6. While operation of the engine is stopped, if an ignition switch 23 is opened, the solenoid coil 15 is deenergized, further intake pipe negative pressure is removed, and the valve element 9 results to open the valve port 6. Then an outer vent passage 7 is communicated to a canister 20 through a passage 18 and conduction pipe 19, and fuel vapor, generated in a float chamber 2, flows into the canister 20 through a primary inner vent passage 4, and is adsorptively held by activated charcoal.