An evaporated fuel treatment apparatus that has a leakage checking means that detects the conditions of change in the internal pressure of the fuel tank when said fuel tank is closed after being placed under a negative pressure, and a judgement means that judges the presence or absence of leakage in the fuel tank on the basis of the detection results obtained by the leakage checking means and correction checking means. The apparatus further includes a pressure variation checking means which detects the degree of pressure variation in the fuel tank, and then opens the fuel tank to the atmosphere and judges whether or not the pressure inside the fuel tank has dropped. Judgement prohibition means is provided that prohibits a judgement of the presence or absence of leakage from being made by the judgment means in response to a judgement by the pressure variation checking means that the pressure has dropped. Accordingly, a judgement of the presence or absence of leakage in the tank system is prevented from being performed in cases where such a judgement might be inaccurate.