By calculating a fuel vapor quantity in a fuel tank based on a fuel temperature and a fuel residual quantity in the fuel tank, then calculating a fuel vapor purge quantity based on an engine revolution number and engine torque when an engine is driven, and then estimating a fuel vapor quantity captured in a vapor capturing unit based on the fuel vapor quantity and the fuel vapor purge quantity, the engine can be driven to purge the fuel vapor to the engine before the fuel vapor overflows the vapor capturing unit to thus be discharged into the air.