Abstract: Fuel vapor from a fuel tank (4) of an engine (1) is adsorbed by a canister (6) via a first passage (9). The canister (6) comprises an air vent (11) which communicates with the atmosphere. The canister (6) communicates with an intake passage (2) of the engine (1) via a second passage (10) provided with a purge valve (13). When the air vent (11) and purge valve (13) are opened, the adsorbed fuel in the canister (6) is purged to the intake passage (2) due to the intake negative pressure of the engine (1). If the negative pressure of the second passage (10) becomes stronger than a reference negative pressure during purging, the purge valve (13) is throttled to prevent an excessive negative pressure from acting on the fuel tank (4).