An evaporated fuel treatment apparatus includes a diagnosis section for leak diagnosis of an evaporated fuel tightly-closed system. In first diagnosis stage in which an atmosphere on-off valve is in close state after an ignition switch is turned off, the diagnosis section performs first leak diagnosis, based on relationship between tank internal pressure detected by a tank internal pressure sensor and first threshold value; and in second diagnosis stage in which the atmosphere on-off valve is in close state after completion of first leak diagnosis, the diagnosis section performs second leak diagnosis, based on relationship between tank internal pressure detected by the tank internal pressure sensor and second threshold value. The atmosphere on-off valve is once opened after completion of first leak diagnosis. First threshold value is set with larger deviation from the atmospheric pressure compared with second threshold value used in second diagnosis stage.