A vaporized fuel processing apparatus has a fuel tank, a canister configured to adsorb vaporized fuel, a vapor path connecting the canister to the fuel tank, a closing valve provided in the vapor path, a pressure sensor measuring inner pressure of the fuel tank, and an electric control unit connected with the closing valve and the pressure sensor. The electric control unit is configured to close or open the closing valve in order to initialize the closing valve or open the closing valve in order to release the vaporized fuel from the fuel tank. Further, the electric control unit is configured to detect failure of the closing valve depending on change in the inner pressure of the fuel tank within a predetermined time from when the closing valve is opened or is closed for initialization of the closing valve or release of the vaporized fuel from the fuel tank.