A leak determining apparatus for an evaporated fuel treatment system is provided for executing a leak determination for an evaporated fuel treatment system without suspending it even in such a condition that variations in fuel could occur, thereby accurately providing a leak determination result. The leak determining apparatus calculates a reference differential pressure difference DDPZ as a difference between differential pressures of an inner tank pressure PTANK detected at two times in a leak check mode, i.e., a differential pressure DPZ2 and a differential pressure DPZ1 detected a predetermined slosh determination time before, and compares the reference differential pressure difference DDPZ with a slosh determining threshold value DDPZG to determine the presence or absence of sloshing. When the sloshing occurs, the leak determining apparatus calculates a second differential pressure DP2 using an inner tank pressure PTANK corrected with a slosh correction value DDPZHOSEI, and executes a leak determination based on the second differential pressure.