A leak diagnostic apparatus for a vaporized fuel processing system that purges vaporized fuel in a fuel tank of a vehicle is disclosed. The leak diagnostic apparatus comprises a leak diagnostic device, a fuel tank condition detection device and a determining device. The leak diagnostic device carries out a leak diagnosis of the vaporized fuel processing system while the engine is operating. The fuel tank condition detection device detects the conditions in the fuel tank. The determining device determines whether the leak diagnosis should be carried out based on predetermined conditions in the fuel tank.