To provide a method for the gateless molding of a thermoplastic resin product which is designed for directly melting and solidifying a molding material in a molding die, and a gateless molding device.

This method for the gateless molding of a thermoplastic resin product is to directly charge a molding material 7 in a cavity 4 on a synthetic resin molding die 1, melt the molding material 7 in the mold 1 by the energy E of an ultrasonic wave applied to the die 1, and mold the molten resin by solidification. The gateless molding device has a cavity formed between a male molding the 2 and a female molding die 3, and also an ultrasonic oscillator 6 mounted on either of both molding dies 2, 3. In addition, the device molds a molding either by oscillating the cavity 4 by an ultrasonic wave transmitted from an oscillator 6 or oscillating 5 part, of the cavity, which forms a horn 5 oscillated by the ultrasonic wave with the ultrasonic oscillator 6.