Abstract: An internal thread forming apparatus includes a housing body fixed on a machine tool, a driving spindle connected to the rotatable main shaft of the machine tool and rotatable at the same speed as that of the main shaft, a movable body movable vertically within the housing body, a rotary lever actuatable to move the movable body downward, and a first return spring biasing the rotary lever to return to a static position. A positive-revolution driving body and a reverse-revolution driving body are carried on and are movable synchronously with the movable body, and rotate in opposite directions. A speed reduction gearing interconnects the driving spindle and the face gears so as to rotate the face gears at a speed less than that of the driving spindle. An output spindle is mounted slidably within the movable body. A tap is fastened to the output spindle. A driving block is carried on the output spindle.