In a vaporized fuel processing apparatus in which fuel vapor within a fuel tank is adsorbed by a canister, the adsorbed vaporized fuel is drawn to an engine, a closing valve is provided connecting the fuel tank and the canister for controlling communication between the fuel tank and the canister, and a purge valve is provided connecting the canister and the engine for controlling communication between the canister and the engine. The vaporized fuel processing apparatus includes an internal pressure sensor configured to detect a pressure of a space within the fuel tank as an internal pressure, and a closing valve control means configured to open the closing valve for supplying an atmospheric pressure to the fuel tank via the canister when the sensor detects that the internal pressure of the fuel tank is negative, while the purge valve is closed. Therefore, the air/fuel ratio is prevented from being disturbed.