A heater for an electronic thermostat, comprising a tube which comprises a hollow structure of which one side is opened and the other side is closed; a first lead line which comprises a first end portion exposed to an outside of the tube and a second end portion placed inside the tube and receives electric power from an exterior through the first end portion exposed to the outside of the tube; a second lead line which comprises a first end portion exposed to the outside of the tube and a second end portion placed inside the tube and is installed to be spaced apart from the first lead line; a heating coil which electrically connects the first lead line and the second lead line and generates heat based on electrical resistance, is shaped like a 'U'-shape, and comprises a first end connected to a front end portion of the first lead line inside the tube and a second end connected to a front end portion of the second lead line inside the tube, the front end portion of the first lead line being inserted in the first end of the heating coil, the front end portion of the second lead line being inserted in the second end of the heating coil, the both ends of the heating coil being connected to the first lead line and the second lead line respectively by electric resistance welding or laser welding; an insulation material which is filled in an inner space of the tube; a packing which is installed inside a mouth of the tube and made of an insulating substance serving as a support for supporting and maintaining the first lead line and the second lead line to be spaced apart from each other; and an insulation pipe which is shaped like a pipe and installed for securing insulation in the first lead line and the second lead line placed inside the tube, has through holes at left and right sides of the insulation pipe, and covers left and right coil portions of the heating coil, the first lead line and the second lead line being inserted in the through holes respectively; characterized in that the heater for an electronic thermostat further comprises an insulation ring provided under the heating coil for securing insulation between a lower end portion of the heating coil inside the tube and a lower end portion of the tube; wherein the insulation ring has a through hole at the center thereof, and the through hole has a diameter smaller than the width of the heating coil. a tube which comprises a hollow structure of which one side is opened and the other side is closed; a first lead line which comprises a first end portion exposed to an outside of the tube and a second end portion placed inside the tube and receives electric power from an exterior through the first end portion exposed to the outside of the tube; a second lead line which comprises a first end portion exposed to the outside of the tube and a second end portion placed inside the tube and is installed to be spaced apart from the first lead line; a heating coil which electrically connects the first lead line and the second lead line and generates heat based on electrical resistance, is shaped like a 'U'-shape, and comprises a first end connected to a front end portion of the first lead line inside the tube and a second end connected to a front end portion of the second lead line inside the tube, the front end portion of the first lead line being inserted in the first end of the heating coil, the front end portion of the second lead line being inserted in the second end of the heating coil, the both ends of the heating coil being connected to the first lead line and the second lead line respectively by electric resistance welding or laser welding; an insulation material which is filled in an inner space of the tube; a packing which is installed inside a mouth of the tube and made of an insulating substance serving as a support for supporting and maintaining the first lead line and the second lead line to be spaced apart from each other; and an insulation pipe which is shaped like a pipe and installed for securing insulation in the first lead line and the second lead line placed inside the tube, has through holes at left and right sides of the insulation pipe, and covers left and right coil portions of the heating coil, the first lead line and the second lead line being inserted in the through holes respectively; characterized in that the heater for an electronic thermostat further comprises an insulation ring provided under the heating coil for securing insulation between a lower end portion of the heating coil inside the tube and a lower end portion of the tube; wherein the insulation ring has a through hole at the center thereof, and the through hole has a diameter smaller than the width of the heating coil.