PURPOSE: To prevent heating coils from thermally interfering with each other and obtain a long life of a heater by dividing heating coils into three for each phase, and arranging and connecting them in tandem in the axial direction in an outer tube.

CONSTITUTION: Three cores 2, 3 and 4 are used for a cartridge heater, and four through holes 21-, 31- and 41- are provided on cores 2-4. Resistance wires are wound around cores 2-4 to form heating coils 5, 6 and 7. Both ends of heating coils 5-7 are inserted into different through holes 21-, 31- and 41-, and end sections 51, 61 and 71 at the lead-out side of coils 5-7 are inserted into through holes 21, 31 and 41 faced to each other on the same axis of cores 2-4. When the first-phase current flows from an AC power source via lead wires 9, 10 and 11, the heating coil 5 wound on the first core 2 is heated, for example, when the second and third-phase currents flow at the phase difference 120°, heating coils 6 and 7 are heated. Three coils 5, 6 and 7 arranged in tandem are circulatingly heated in sequence, and an outer tube 1 is wholly heated from the tip to the base section.