A fuel vapor processing system includes a canister, a tank passage communicating between the canister and the fuel tank, and a purge passage communicating between the canister and the intake air passage. Fuel vapor adsorbed by the canister can be desorbed and purged into the intake air passage via the purge passage due to a negative pressure produced in the intake air passage. A desorption promoting device can promote desorption of fuel vapor from the canister. A control unit controls the desorption device, so that the desorption promoting device promotes desorption of fuel vapor from the canister during desorption through the purge passage due to the negative pressure.