Abstract: A safety plastic filler neck for an automobile, vehicle, etc., that functions both to control the screwing torque and to prevent the fuel leakage from filler port through releasing a cap separable from the screw body. The screw body includes a flange disposed on its top portion, engaging/disengaging teeth formed at several locations along its outer circumference by extending the flange, a circular standing edge provided on the flange, latching teeth formed at several locations along the outer circumference of the circular standing edge which are to be latched in screwing direction, and incisions provided under the latching teeth and corresponding to them. The cap includes a rotation flange disposed on top of it, a round cover edge for covering the circular standing edge and the engaging/disengaging teeth of the screw body, latches formed on upper inner surface of the round cover edge, and an anchoring groove formed in lower inner surface of the round cover edge.