A heat sealer comprising: a press mechanism; a heater attached to a jaw of the press mechanism; and an electric power circuit connected to the heater, the electric power circuit receiving electric power from an external power source, wherein the heat sealer clamps and presses an object made of a thermoplastic resin by using the press mechanism, instantaneously heats the heater by passing a high electric current through the heater in a short time, melts the object, and cools and solidifies the object by interrupting the electric current, wherein the heater includes (a) a thin thermal-electrical insulator plate that has an appropriate size and thickness and that is superposed on a heat dissipating base or on the jaw that also serves as the heat dissipating base, the heat dissipating base being made of a metal and disposed on the jaw of the press mechanism, (b) one or more heater wires each of which is a thin metal resistor plate superposed on the thin thermal-electrical insulator plate, each heater wire including an electrode that is connected to the electric power circuit and a heat generating portion having a strip-like shape or a desired shape, (c) a thermally conductive electrical insulator, which is a ceramic plate, superposed on the heat generating portion, the thermally conductive electrical insulator having a flat surface that closely contacts the heat generating portion, the thermally conductive electrical insulator being an electrical insulator and having a thermal conductivity that is equal to or higher than that of alumina, (d) a thermal-electrical insulator that covers exposed portions of the heater wires and the electrode, the thermal-electrical insulator surrounding a part of the exposed portions located around the thermally conductive electrical insulator so that the part is electrically and thermally insulated and surrounding a remaining part of the exposed portions so that the remaining part is at least electrically insulated, and (e) a thin release film superposed constantly or as necessary on the thermally conductive electrical insulator with or without a gap therebetween, and (f) wherein the elements of the heater described in (a) to (e) are made to closely contact each other and fixed at necessary positions by using an adhesive, a sticky agent, or a physical method. a press mechanism; a heater attached to a jaw of the press mechanism; and an electric power circuit connected to the heater, the electric power circuit receiving electric power from an external power source, (a) a thin thermal-electrical insulator plate that has an appropriate size and thickness and that is superposed on a heat dissipating base or on the jaw that also serves as the heat dissipating base, the heat dissipating base being made of a metal and disposed on the jaw of the press mechanism, (b) one or more heater wires each of which is a thin metal resistor plate superposed on the thin thermal-electrical insulator plate, each heater wire including an electrode that is connected to the electric power circuit and a heat generating portion having a strip-like shape or a desired shape, (c) a thermally conductive electrical insulator, which is a ceramic plate, superposed on the heat generating portion, the thermally conductive electrical insulator having a flat surface that closely contacts the heat generating portion, the thermally conductive electrical insulator being an electrical insulator and having a thermal conductivity that is equal to or higher than that of alumina, (d) a thermal-electrical insulator that covers exposed portions of the heater wires and the electrode, the thermal-electrical insulator surrounding a part of the exposed portions located around the thermally conductive electrical insulator so that the part is electrically and thermally insulated and surrounding a remaining part of the exposed portions so that the remaining part is at least electrically insulated, and (e) a thin release film superposed constantly or as necessary on the thermally conductive electrical insulator with or without a gap therebetween, and (f) wherein the elements of the heater described in (a) to (e) are made to closely contact each other and fixed at necessary positions by using an adhesive, a sticky agent, or a physical method.