A vaporized fuel treatment device includes a sealing valve disposed in a vapor passage between a fuel tank and a canister. An ECU is programmed to decrease an internal pressure of the fuel tank by a pump via the canister so as to diagnose whether leakage occurs in the vapor passage in a state where the communication between the canister and the atmosphere is cut off and the sealing valve is opened. The ECU is programmed to execute a learning of a valve opening start position of the sealing valve based on a change in the internal pressure of the fuel tank when changing an axial distance between valve element and a valve seat when the internal pressure of the fuel tank is equal to or smaller than a predetermined value after a diagnosis of leakage in the vapor passage.