A method of continuously manufacturing a hermetic plastic zipper wherein the plastic zipper has a readily peelable plastic layer prolonging in a lengthwise direction of the plastic zipper, the readily peelable plastic layer is welded using ultrasonic wave so that the peeling intensity of the welded portion is within a certain range, the method is characterized in that while the engaging portions are brought into a engaged state, the portion of the plastic zipper where the readily peelable plastic layer exists is inserted between an ultrasonic horn and an anvil, and the readily peelable layer is welded by using ultrasonic wave under a condition wherein the distance L between the ultrasonic horn and the anvil is uniformly held at an distance of H≧L≧0.85H wherein H stands for a maximum thickness of the plastic zipper under the engaged state.