An image forming apparatus comprising: an image forming portion (15, 16, 17, 18, 19) for forming an image on a recording material; and a fixing portion including a tubular film and a heater (300, 600) including a substrate (305, 605), a heating element (302, 303, 602a-1-602a-7, 602b-1-602b-7) provided on the substrate, and a temperature sensing element (T1-T7) provided on a surface of the substrate opposite to a surface on which the heating element is provided, wherein the fixing portion is configured to fix the image formed on the recording material to the recording material with heat from the heater which is controlled according to a sensed temperature by the temperature sensing element, wherein the image forming apparatus includes a temperature sensing circuit (403, 703) to which the temperature sensing element is electrically connected, a surface of the heater on a side where the temperature sensing element is provided faces an inner surface of the film, the heating element is provided in a primary side circuit (401, 701) which is electrically connected to a commercial power supply, characterized in that between the primary side circuit and the temperature sensing circuit, basic insulation or reinforced insulation or double insulation in which additional insulation is added to the basic insulation is provided, and between a secondary side circuit (402, 702) which is reinforced insulated or double insulated with respect to the primary side circuit and the temperature sensing circuit, the basic insulation or the reinforced insulation or the double insulation is provided, and the temperature sensing circuit is a circuit different from the primary side circuit and the secondary side circuit. an image forming portion (15, 16, 17, 18, 19) for forming an image on a recording material; and a fixing portion including a tubular film and a heater (300, 600) including a substrate (305, 605), a heating element (302, 303, 602a-1-602a-7, 602b-1-602b-7) provided on the substrate, and a temperature sensing element (T1-T7) provided on a surface of the substrate opposite to a surface on which the heating element is provided, wherein the fixing portion is configured to fix a tubular film and a heater (300, 600) including a substrate (305, 605), a heating element (302, 303, 602a-1-602a-7, 602b-1-602b-7) provided on the substrate, and a temperature sensing element (T1-T7) provided on a surface of the substrate opposite to a surface on which the heating element is provided, wherein the fixing portion is configured to fix the image formed on the recording material to the recording material with heat from the heater which is controlled according to a sensed temperature by the temperature sensing element, wherein the image forming apparatus includes a temperature sensing circuit (403, 703) to which the temperature sensing element is electrically connected, a surface of the heater on a side where the temperature sensing element is provided faces an inner surface of the film, the heating element is provided in a primary side circuit (401, 701) which is electrically connected to a commercial power supply, characterized in that between the primary side circuit and the temperature sensing circuit, basic insulation or reinforced insulation or double insulation in which additional insulation is added to the basic insulation is provided, and between a secondary side circuit (402, 702) which is reinforced insulated or double insulated with respect to the primary side circuit and the temperature sensing circuit, the basic insulation or the reinforced insulation or the double insulation is provided, and the temperature sensing circuit is a circuit different from the primary side circuit and the secondary side circuit.