An image forming apparatus includes a thermal fixing unit having a fixing member heated or cooled at multiple levels of the target fixing temperature in accordance with prescribed different image formation conditions, and a pressing member pressing against the fixing member to form a fixing nip thereon. The invention provides a method for calibrating a pyrometer by means of a cold source which is also applicable to processing chambers with a narrow slit. A plurality of paths is connected between the storage 1 and 2. The thermally conductive sheet is disposed so that the insulation layer covers the terminal of the electronic component and the electric circuit. 40\u2266Re\u2266130\u2003\u2003100\u2266Rth\u2266200\u2003\u2003Re is an in-plane retardation value for light at a wavelength of \u03bb nm, and Rth is a retardation vale in a thickness direction for light at a wavelength of \u03bb nm.