Abstract: A method of automatically controlling the fusion process of thermoplastic resin articles is done in a stepwise fashion. In response to a start signal, a current is applied to an electrical resistance heating element for fusing together facing resin surfaces. Electrical characteristics of the resistance heating element are calculated upon increase of the power to a given level and the specific heat of the resin is calculated through ON-OFF control of the resistance heating element. The power is then raised through stages based on the electrical characteristics of the resistance heating element and the specific heat to determine variation coefficients of the characteristics and specific heat at each stage and through further ON-OFF control. Finally, the power increase is stopped as the variation coefficient of specific heat reaches or becomes smaller than a given value.